Re: scored cyl wall
That piston is damaged due to detonation and at this point it looks relatively mild. If that cylinder still has compression--say above 90-100 PSI, try a light honing and a new piston. HINT: These pistons are not available HOWEVER, you can buy a piston for a late model FORCE 75 or 90 (3.375 bore std.) from Wiseco and it will fit. You only need to modify it by drilling two holes in the skirt for gas flow.
When you reassemble, be certain to go over BOTH carbs and make sure the top one does not go lean again.
The engine was probably the BEST design Chrysler ever made and is a very strong runner--for that era. It was the epitome of cross-flow design. Shame it was never extended to the larger three and four cylinder engines.
BTW: Magna-Power II was a very poor, trouble prone ignition. If yours keeps running, you will be one of the lucky ones. If it fails, consider retrofitting to the Prestolite or sell the engine.