A 40T rear sprocket is a good start. Mine will do about 30 on a flat. A little earlier tonight I saw 34 on a slight downhill. i think getting a little 50 up to 40 mph will be difficult, but not impossible. An expansion chamber would probably be next on the list.
give it some time to break in, after a tank of gas or two it should run better. then tune in your carb by rejeting, will most likely need to lean it out some with a smaller jet. and an expansion chamber exhaust like bikenut said is always the best bang for the buck in motorbike mods.