These little bikes can be deceptively fast and you can get considerable performance gains by upgrading and increasing the displacement of your motor. Adding a big bore kit will require you to disassemble your motor and replace your piston and cylinder with a larger set. Modifying a 50cc pocket bike with a big bore kit to a full 70cc and adding an aftermarket intake and fuel system will help you squeeze past the 65 mph mark.
Add a big bore kit to increase speed. When your motor is disassembled, have the cylinder and cylinder heads ported and polished. This will maximise the volume of fuel and air necessary to boost power.
Upgrade your stock intake/exhaust set-up. A high-performance aftermarket exhaust and free flow air filter will provide a better flow of air. Installation of a high-performance carburettor will allow you to fine tune the air-to-fuel ratio.
Install an after-market exhaust system. It will give your bike more power through all of the RPM range and its wider and longer expansion chamber allows the exhaust system less back pressure.
Take your bike on a short test run to make sure everything is running smoothly. Once you're confident in the bike's performance, test your speed.
Remember with every part you add, there has to be a balance and re-tune.