You could square off the end of the crank case to make sure it is square, and then remove material from the BACK of the prop drive washer. When you see that you have some satisfactory clearance, see how it tightens up. If you have clearance still, you can lap the drive washer to the crank shaft taper with fine lapping compound. Mount the engine in a test stand, mount a prop on it, take the glow plug out, and check how the prop runs out at this point. If the prop doesn't run straight, then the tapers are not square and straight. This is about the only thing I think you can do with out a lathe and some technical help. You can look for a similar parts engine on eVilBay. A later model crank and drive washer may work but may have to be fitted also.
Good luck and have fun,
Dan McEntee