I know it's tough especially when you want to go on a big trip. I think you're at a junction with some hard choices.
Choice 1:
Fix it properly. That mean tearing down the engine, checking the rings, pistons for wear, changing the valve springs most likely and re-lapping the valves. After that re-jetting will be simple but will still require time/and or jets and new petcock. You will need a top end gasket kit including new valve seals which may run about $200 or so. Reliability high. Cost high. Resale value will be high so keep those receipts! That's assuming it can be fixed properly.
Choice 2:
Fix it sort of halfway using whatever you can to fix it cheaply may be messy if you use say for example a liquid gasket instead of real ones. Make it work just enough to function, beat the hell out of it because you're just going to sell it for parts or as a fixer upper. Cost low reliability low.
Choice 3:
Sell it off now for parts and use what you have plus some of your own money to get another bike in good working order before going on a large trip. Least amount of work. Could be pricey if you don't get much for the bike you have. Cost medium/high Reliability high/depending on what you buy, could be buying another set of problems a bit of a risk but if you're current bike can't be fixed without major work, might be for the best.
It's really up to you, if you feel that half fixing it will do you for the trip then go for it. If you want to totally strip it down and rebuild it to get a good resale value you could go that route. For myself I would either fix it completely or just sell it and get something else. I know it's tough, but do you want to be broken down somewhere in the wilds of Africa with a dead bike and no money to buy another? It's up to you, since you drive it daily you would know way better than any of us how your confidence level is. I would have another go at it at least, see what the damage or potential for fixing is then decide. I've had to sell bikes that I knew were too much money to fix up so I know how it feels, but sometimes you have to cut you losses. Or on the other hand if you love the bike and want to keep it for at least a year it may be worth it to fix up. If the valve springs are slightly phucked it's not that hard to fix, it's mostly just labour and the cost of the gaskets/springs and what maybe $5 for lapping compound. If the pistons and rings are in good shape it will basically drive like new. You can do it in a weekend with a buddy, just supply some beer and have fun. Just take a few days and take another look and sit down with a beer or two and have a no bullshit assessment for yourself and your safety on a long trip.
If you want when you check for the hissing sound put your ear right up to the exhaust side of the bike as well as the intake side near the boots. After that check near the bottom of the crankcase to see if you have that hissing noise. If the pistons as you said looked in great shape as well as the piston rings are in spec. My guess is the valve springs are dead and the valves aren't sealing properly. If the hissing is at the top end then that is most likely the problem. If it's at the bottom then your rings are dead. If it's the valve springs you're in luck as it's not that hard to fix, mostly just work. Under $400 for sure including all gaskets. Once the valves are sealing properly it will be super easy to fix the jetting, probably just a notch or two down on the needle and a screw adjustment. But find out exactly where the leak is by listening to the top of the engine and the bottom. My guess is as the cylinders and valves are heating up, the metal expands and allows a small leak to form, killing compression. If the rings were phucked it would do that from the first second you turned the bike on, not when cold or hot. Try with your ear to hear whether the hiss is at the top or the bottom. If it's the top and the exhaust valves are leaking you will definitely hear it at the exhaust pipe and it will have a hollow sound like blowing through a beer bottle or a tube.
Put it this way if you do fix it properly that $400(probably more like $250-$300) may turn into $1000+ at selling time. If the bike is dead you may have alot of trouble trying to get rid of it and may only get a few hundred bucks compared to $2000 if you keep all receipts and fix up the jetting properly which is super easy. I did it on a GS750EF because I wanted to drive it another couple of years and I kept the receipts and actually sold it for $500 more than I bought it for so it worked out well. After 20 years I would wager the springs are tired, mine were. When we took a look at the rings there was hardly any wear at all and all were well within spec. The springs are the weak point in the engine, as long as the oil is good it will do it's job well and the rings will be fine. While we had access to the head we got all the coking out of there and polished the piston crown and when put back together was literally almost a new bike. If the springs are dead and not sealing, NO AMOUNT of adjusting the valves will fix it.
If you do want to fix it we can help you out, it's not hard just a bit of labour. I might be able to help with some tips as I did it myself. The springs are a no brainer, the key is making sure those valves are nicely lapped and cleaned so you get a good seal. I highly recommend getting a friend to help you out, it makes the time go by quicker and reduces the labour in half, also when taking out the block and popping it back in there it makes it alot easier. You may be able to squeeze the block out of there without pulling the engine if you're lucky. Since I did it over the winter my friend and I pulled the motor but it might not be necessary for the 400 as it's a smaller engine. If you're working solidly you can do it in a weekend or less. The hardest part is putting the block back on and manually compressing the rings one layer at a time to feed the pistons back into the block, you definitely need an extra pair of hands or two but only for like 15 mins or so then your set.
This is what you need for your own info and decision making:
At least one or two cases of beer(most important) $35
A valve puller, basically just a wooden or plastic dowel with a suction cup on the end $10-$20
a good tool set, which you probably have
valve lapping compound $5-$10
top end gasket set $100-$150(if doing it on the cheap may be able skimp if you don't care in a year, or gaskets in good condition)
new valve seals another: $50-$75 (if not in top end kit)
Optional bottom end metal gasket $30(I didn't bother as it was in good shape and had no leaks afterwards)
Some light steel wool to clean up piston crowns and top end/valve coking(be very gentle)
Set of valve springs: $100-$150
About 10 hours labour, less with more people. With enough people, could probably do it in 6-8 hours
Optional ring compression tool: $50, can use your hands if you have enough people to help feed pistons and rings into block when assembling.
Here is the options with this sort of fix comparing cheap/full fix:
Cheap/minimum fix:
With this fix you are fixing the problem but skimping on the top end gaskets, which may leak but are super easy to change out as when the next valve adjustment comes along you'll be changing them anyway. Same amount of labour, maybe save 40 mins by not changing out the gaskets.
Beer: $35
Valve puller: $10-$20
New valve seals $50-$75(skimping is not an option!)
New valve springs: $100-$150(skimping is not an option!)
Valve lapping compound: $5-$10(skimping is not an option!)
Light steel wool
A good tool set
Labour: 10 hours or 6-8 with enough people
A few extra pair of hands
