Had this just happen to me on my 2000 Johnson 9.9
Used to have 95/95psi, then overheat. Still ran (after replacing powerpack) but could not get above 1/2 throttle. Compression @ 60psi
so I pulled head. Head was warped a good .015" so I flattened that on my tablesaw with silicon carbide paper. I also flattened the case. Put a new head gasket in and I am back to 95psi and have full power at WOT again.
Your pistons will rock and twist in the cylinders. This is normal. Side to side / up-down and rotate. As long as your walls do not show major damage, I would try the flattening and replace head gasket. At most that'll cost you $20.
To flatten the case, since I was worried about contamination, I made two cardboard cutouts each just slightly larger than the cylinder bore. I positioned the pistons evenly and pushed the cardboard into the cylinders. Then I smeared grease all around the edge of the cardboad. I took an 8" section of marble trim tile I found at Home Depot (it was flat) and glued 120 SiC paper to it. Then I sanded the case until all the surface was shiny. Wasn't off my much, maybe .003 - .005", Then I repeated with 320 SiC paper.
Then I used my pinky to remove most of the grease, then needlenose pliers to remove the cardboard. As the cardboard is removed, it will squeegee out the grease and contamination. A quick clean with a towel and no contamination.
Good luck!
And yes there is a .020" overbore piston/ring set available, but try the cheap route first - you might be pleasantly suprised.
