RSE is definately the way to go. I started off with their 70cc with high comp head and Speed carb. Have since added their reed valve kit and SBP pipe. Also built a CDI to replace the stock unit.
Build quality is very good with upgraded hardware and bearings and all sorts of extras that you'll only start appreciating once you open the motor to do further mods, e.g. they trim the bottom of the piston at the inlet port.