The rings should go in without excessive filing for end gap of .012″. The side gap with the ring installed should be .001″ or less. Put an allen wrench in the starter nut and pull it through, it should fight you pretty good. If it pulls through kinda easy that might be it but, I suspect ignition.
Did you try a new plug? You could pull the flywheel and check for a sheared key. Are the ground connections clean? Try a different coil.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience