Hyway sells a titanickle 391 cylinder with pop up piston for under 150 on the fleabay channel, thats the route I would take . Just be aware that you likely need to rebuild the carb and pressure/vac test it.
That piston damage could have been caused from a intake leak, improper functioning or ill adjusted carb, damaged flywheel key or bad coil so be very aware and pay attention when you run it after rebuilding it. Aren't some of these known to have the plastic intake plate crack? I thought I read about it a couple times where they caused engine failures slowly over time.