need Your EXPERTISE...
If you are 100% sure the engine is OK, Be sure your clutch shoes are installed properly, you could have put them on backwards. The engine spins counterclockwise and the part that installs on the pin spins first with the meat of the shoe trailing it. The slipper is the small metal disc behind the spur gear held on with the spring and nut on it. You can tighten it down all the way and back off 1/8th turn. If the slipper is too loose the spur gear will be very hot after a few runs. Also it will take a couple tanks for the shoes to seat properly and give you a good launch. You will defanitely need to retune after the modifications.