As I never had a bike w/20" wheels I can't give you specifics and TBH much depends on environmental variables such as engine power output (modded/stock), bike, cargo, and rider weight, terrain, and ofc wheel size as you've said. While you can always compromise acceleration for top speed or vice-versa - the problem remains that you're compromising. Finding that "sweet spot" between the two tends to take a LOT of experimentation due to the variables above.
While no doubt someone more knowledgeable than I can give you a good recommendation for which sprocket will getcha in the ballpark - I really recommend considering a shiftkit
if ya wanna get the most outa yer bike.
Still, just a sprocket swap is pretty easy (partic if you have a sprocket adapter
) and I wouldn't worry too much about the clutch. Not only are the pads replaceable but you'll find that you'll just pedal a bit more to start off, not such a bad thing to hafta do