I have a YM100 Champ that the previous owner miss timed and bent a valve. I'm in the process of reassembly and there are no timing marks on the magneto rotor. The rotor has the Yamaha logo and part number stamped on it so its not aftermarket, I had another set of eyes look and it appears to not have been stamped.
How do I set the cam timing with out that mark on the rotor. Is setting the piston at TDC sufficient?

 

Yes, setting by the piston is OK as long as you get it right. It is really difficult to set within 5 degrees by eye, a dial test indicator on the piston before fitting the head, and marking the flywheel by finding the mid point between where the DTI stops and starts to move, is the best way. However doing your best through the plug hole, preferably again using a DTI, will get it within a tooth or two of right.