I assume you have already removed the obvious including the carburator. Then put the engine in the oven at 200 degrees for about 15 minutes. The liner will then slide right out of the top of the case leaving the piston dangling. Notice where the pin and ring gap align at about 4:00. Then you can "walk" the piston rearward at the wrist pin and off of the crank pin. Just reverse for assembly paying attention that pin in the ring groove and ring gap or alignment notch are aligned properly on piston so the orientation with the liner is returned.

Take care, Cliff