Pull the carb, inspect the tiny filter behind the fuel pump. There is a chance damage has already been done. If the filter is clogged, the engine has run lean. If this goes on long enough, the piston tops will usually grenade in some fashion and the whole engine will need to be pulled and rebuilt from the crank up. Also, I think your motor is a 720cc, it's also pretty common for the rotary valve clearance to become too large due to wear and will need to be machined to bring it back into spec. This can also cause some wierd running issues.