Just to clear a few things up:
- The crankcase, head button, and bearings are the only carryovers without modification.
- Pinch. The GX series is still pretty tight on the top end. You will need to heat up to 190-200 degrees Fahrenheit. This will also help when running rich during breakin.
- Break in. I've seen several different techniques on here, and they're all pretty sound. I do like Rex's approach at preheating to 240 deg (F), and then turning over. I recommend going to at least 190, but do not exceed normal running temperature. If the piston gets stuck at the top, heat it up more.