I have seen broken piston rings be the problem.
A few years ago when I was working on small aircraft we had a plane come in for its annual checkup. This aircraft flew in on its own and though engine ran, it was not as powerful as it should be. We ran a compression test and found only one cylinder with a low reading, it read 0, we even looked through a bore scope and did not see any obvious problems. We removed the problem cylinder and found that the piston rings had broken into many pieces.
After you remove the head and if it checks out fine and the piston moves in the cylinder then have it removed to see what condition the rings are in.
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