I recommend an expansion chamber, a high performance air cleaner, some carb jets, and if you don't already have one, a 36T rear sprocket. Just that alone will get you to 35-40 mph which is already faster than most motor bicycles should go. If there's hills where you live consider a 40T sprocket. You'll still be able to get 35 mph.
The SBP pipes works well, and the one mentioned above gets good reviews too. Pirate Cycle sells sprockets, and hub adapters.
These mods are very easy to do and won't have much effect on overall dependability.