lots of thing you have to consider before you decide the size of rear sprocket...
1. Engine Modifications
2. Weight
3. Your preference <--( highway or town setting)
Instead of complaning on the engine performance, better you learn to feel, understand or butt dyno your engine. From there you will know how to decide the correct sprocket. Outsiders only can give you guide but not exact size.
the most important thing on 4stroke is the head, a very good PnP and bigger valve size and followed by others bigbore, carby, gearbox ratios, exhaust, cdi, valve spring, clutches, coil, stroker, etc will give you more speed...
my 2 cents