You can change the gearing on it,also if you don't care about the lifetime of the engine you can tune it and make it run lean for more performance.making it lighter will help,strip off everything you don't need. I do not recommend tuning the engine too lean though, it will be good to tune the carb though.
The SSR 125 CC pit bike is one of the best bikes to increase your top speed. It has 7.8 hp and a top speed of up to 50 mph. You can also work with your bike but make some adjustments on your gear. You can check with your mechanic for more inquiries.