I would be looking at simple things first, such as your timing and points as suggested above. If the unit has been sitting I would pull the carb off and go through that as well. Make sure the fibre washer is in place where the jet tube seats down the middle of the carb. Make sure the jet is clear and while your there check your float height, 3/16" from the base when inverted.
Things to try that cost nothing :-)
Drain tank, fresh fuel
Try another spark plug
Check carb over
Check spark and timing
Good luck