For me satin black, seems the best option because of availability of the color. You can find it anywhere and really since the motor is watercooled and in the breeze, you can get away with a regular rattle can versus actual "engine" paint. I usually just spray the area down with brake clean to get the funk off, wipe clean and spray. Done.
The motor in the Bandit now is that bronze metallic OE Suzuki but it's defineately a 10 ft rule kind of thing. It's looking like it's 13 years old. I know walking down the aisle of Home Depot or even Pep Boys, I've seen some close cousins in the paint section but they're more expensive versus Rustoleum satin black.
My .02$$
-Randy