I know most boat owners hope the engine will outlast their ownership , but sometimes a rebuild becomes necessary.
The gas folks have it easiest , a new short block, move the "marine" stuff and its year one again.
For years auto guys would rebuild to gain whatever "performance" the were looking for.
Here is a source of Diesel rings that might work better than the factory stuff , should someone get involved enough.
No reason the lighter duty matinizations cant be made more efficient.