I have collected old 2 cycle outboard boat motors. When I get one set up I use PB Blaster. Let it soak a day or two and then just keep trying to turn the engine over. I have never yet not have one not turn over in a day or so. Also I have found with a good spark they will start right up and I have yet ever did anything with the piston or rings. They just run fine. If it was me I would try to start it with the old piston before pulling it apart and don't be surprised if it runs.