more power

What's the best to get more power out of a standard st 70 (1972) with 20000 miles on the clock ,it pulls quite strongly and can get almost 40 out of it on a long flat road . has anyone tried a big bore kit ? ,any help would be great thanks

This is the most common mod on the ST70 engine:
- 85cc (51mm) cylinder & piston
- new gasket kit
- original or 16-18mm carb, possibly with another air filter
- preferably a 180%-210% oil pump for 6v casings
- check the version of your ignition / flywheel. This will limit the top speed / rpm's.
The flat Hitachi can't be changed for a faster one.