Hold that thought-I may well be wrong.
Turn the engine over and look for damage/scuffing/scoring on the bore. It may be that someone has reused the head after the rings were replaced. There's usually lumps of ring stuck in pits like that.
If you can't see anything then put it back together and do a compression test.
<edit> I enlarged the pic btw -I think I can see bore damage on the lower end of the closeup photo of the pistons so don't be too hopeful.
It looks like you may have a blown crank seal too-it's not usual to have one piston cleaner than the rest unless it's letting air or water in on that pot and running too hot.
Need spares,consoles,consumables,hire,training or even a new boat?
Please click HERE
and support our Trade Members.
Join up as a Trade member
or Supporter HERE