-Shed Weight
-Properly tune your carb
-Properly Sized Sprocket
-Proper fuel/oil mixture
-Tuned Exhaust
-Port Match Intake/Exhaust
-Increase Compression
-NITROUS!
-Various advanced machining techniques which you can find in this forum.
There's lots of things to do but you must realize the limits of the powertrain you are working with. How fast do you really want to go on a bicycle? I get scared anywhere above 30mph. My advice is read most of the threads in this forum and decide what your goals are, whats realistic, and whats within your budget.
Be forewarned, once you get a taste of more power out of the china girl your gonna want more, and more, and more.
Hope that helped, at least a little.