RE: Black piston?
Soak it in automatic trans fluid and put it back together and see if it will run. If it runs keep running it. It it doesn't junk it and buy a Super Tiger 51 or buy another Thunder Tiger. Forget buying parts as they are too expensive and by the time you replace the bearings the piston or something else will go out. If you replace both the bearings and piston it is too close to the cost of a new engine.