Well #6 looks like a problem-I had about the same problem on a old VW dune buggy were one of the pistons melted down one side 'cause the rings end gap lined up (or I screwed up) and the hot exhaust gases eroded the piston away. What you need to do now is find out if it is valves or the piston/rings. Do you have a air compressor? You need to put #6 at TDC (same as #1) and put air into it using the spark plug hole. You may hear it coming out the intake/crank case.
