1. Remove the spark plug. This will allow you to access the piston.
2. Pour a small amount of oil or solvent into the spark plug hole. This will help to loosen the transmission oil.
3. Insert a brush into the spark plug hole and scrub the piston. Be sure to remove all of the transmission oil.
4. Wipe the piston down with a clean cloth. This will remove any residual oil or solvent.
5. Reinstall the spark plug. This will complete the cleaning.
_Note:_ it is a very bad idea to use too much oil that would cause engine destruction you need just oil enough not to ruin and waste oil. Be safe.
Also be aware that a professional inspection may still be needed by a licensed professional if there has been too much damage.