Dumb question time: does the piston move up and down in the cylinder (with the head off) when the kickstarter is pushed? Note: I ask with the cylinder on because if you try this stunt with the cylinder off, the rod / piston will move at a surprisingly high RPM off of the kickstarter. It is possible to damage the rod or the piston (assumng the piston is OK) as they go flailing about.
You say the piston, rings and bore "look" fine. What are the clearances? Can your eyeballs tell the difference between a .002" piston-cylinder gap and a .003" gap? How about ring end gap and ring profile? Does the cylinder have a fresh hone to de-glaze the cylinder walls?
Once you establish those specs as OK (per the manual), then maybe look at the auto clutch system. If the autoclutch is slipping, that might cause the kickstarter to not fully engage.
