Hey man6ano
Welcome to the board...
The cheapest option is to swap in a 2nd gen B12 shock. I have one in my bike. It's NOT a big upgrade but it does have a slightly stiffer spring, and the shock is a little higher quality, but not much. Just like the B6, it has rebound and preload adjustment only. Look for one on E-Bay or find a B12 guy who has upgraded to a Busa shock and work out a deal.
The shock is a little longer, but less than 1/4" IIRC, it raised the rear of my approx 1/2", about the same as dropping the front, which is commonly done. 2nd gen B12 is 2001-2005 in the US.
You can also swap in the Busa shock, like on the B12. It will fit, but will raise the rear more than it does on a B12. Likely about 1-1/2". If you go this route, you will want to ensure that the front fork caps are flush with the triple trees (ie forks fully extended). If you find the bike a little too skittish, you can also change from the standard 120/60 front to a 120/70 front, which is about a 1/2" taller as well.
On my setup, I have the B12 shock, forks dropped 5/8", progressive fork springs with approx 1-1/4" preload, and a 120/70 front tire. This is a good compromise for stability and quick turn-in. I do a lot of longer highway trips, but on tight roads, so this is how I like it. If I were setting up the bike for around town, I would stay with the 120/60 front.
Hope this helps...