Jesse There's a rule of thumb, not everyone agrees but most do, and that is to wait until the engine is broken in before starting to add go-fast parts. You'll need to get a couple of hundred miles on the engine before it would be considered 'broke in'.
After that go for one modification at a time. That way you'll have some basis for comparison to establish a base line from. If something doesn't work, you'll know where to start looking.
Rear sprocket size, the smaller the faster, is one place to increase speed. Expansion chamber exhaust and proper carburtetor tuning are other places to start.
There are bikes that will reach into the 50mph range but we typically advise against trying to achieve those speeds unless you've done serious up grades to the bearings, wheels, spokes and especially the brakes. Frame construction is also important.
You have to keep in mind that you're building a bicycle, not a motorcycle. 25 to 30 is a much more reasonable goal and more easlily achieved. Safety is also a prime concern. When you start getting above 40mph you're getting into an area that can get you seriously hurt unless you take precautions.
Do some reading here in the 'High Performance' section to see what others have done to their bikes when getting into the motorcycle speed range. Take it from an old hot rodder, bicycles were never meant to keep up with traffic. Even motorized bikes.
Good luck. Have fun but ride safe.
Tom
