Small Engine Troubleshooting Problems? Ask a Mechanic Now.
You can do the upper end of the engine in the ATV. Cylinder head will be removed, and then the cylinder can be removed and lifted off the crankcase, which exposes the piston, Remove the piston pin retainers and push the piston pin from the piston.
Here is link to CD service manual. Or get you a Clymer manual for the unit.