If you do pull the engine apart, start with the cylinder. Carefully inspect the ring to make sure it is free on the piston. These rings are fit very close in the ring groove and are prone to stick which could cause your problem. These engines should have a very strong compression feel when the prop is pulled through. Quite a bit more than most more modern engines. If stuck, freeing up the ring might resolve your problem. These engines are getting some age on them now so any number of problems might be found with a good teardown and reassembly but no need to go much further than necessary if it is a simple problem.