i'm in the same boat, i don't need to go above 35mph, but i would like to get there quicker.
lot of these guys on here have moved up 60cc kits, jemco pipes, and bigger carbs. they can go 40 to 50mph.
the pw50 gears will help with your top end, not your low. for better acceleration you should keep your stock gears and replace your clutch springs with racing springs. you could also shave the head back a little, get some power reeds, read about port matching too.
a good after market air filter and upjet is always nice.