My .02
You can run as much bore as you're willing to try, but head gasket life fades quickly over 57-58mm, with the long spindly studs. 60-61mm isn't uncommon. Reliability there, is. I really like just boring them to 57mm for the TB lightweight piston, adding an offset pin and stroking to 66mm. you can get a 3mm spacer and swap to 14/28 sprockets, or 3mm longer rod, mm spacer and 92l cam chain (Yamaha 125 Breeze ATV).
Make sure you've plotted the ignition curve or at least done the retard mod. 26-28 total is all a good hemi needs. More only adds low speed snap, and kick gear snapping. Stock thin-shell outer rotor is decent, or Lifan adjustable for an upgrade.
Head is a restriction at 150cc, worse as you go up. It needs bigger valves, a good valve job and some porting. There's the silly quench-killer step in the chamber, too, to deal with. Adding cam duration will let it breathe on top and kill off some kick resistance and slow-speed tire spinning.
