porting helps a lot. on the intake, dremel out as much as you can to match the intake port on the engine. be careful not to go overboard. the engine port is bigger than the welds on the intake. while you're grinding away, get up inside of the intake manifold and smooth out the casting burrs.
the exhaust can get the same treatment, but again, there's not enough material to match the port exactly, so just do as much as you can.
pay attention to your spark plug's color till you get that dialed in.
make sure your bike's tuned up. good tires, straight wheels, no loose parts, greased bearings, etc. will help. the less rolling resistance that motor has to push, the faster it'll go.
as far as sprockets go, it depends on your weight and where you ride.
i'm 135 and i've got a 32 sprocket. works great on the flats, but bogs on steep hills. i can get an average of 37mph.
i had a 36 which worked great overall, and it averaged about 34.
nos and boost bottles are just gimmicks, don't waste your money. buy a tuned pipe instead. (haven't tried one myself, but everyone else swears by them.)
whenever you're adding a part, tuning something up, or modifying something, do it one piece at a time. that way you'll know what actually works for you and what doesn't. it'll also help you find problems. if you do ten mods at once and your bike stops running, you've got ten times the problems to figure out why.
despite what people say, don't expect much over 35mph, unless you're willing to spend money and tons of time fine tuning it. everyone likes to believe they're getting more out of their cheap little engine than they really are. (including me