803A Hits a Wall at 9.3kW

Might I suggest that you add a bit of seafoam to the oil before changing it.
It does a great job of cleaning inside the engine and hopefully freeing up piston rings.

Yes it’s safe for that, says it right on the can.
Just can’t run it forever.
I don’t remember the max amount of time it can stay in before dropping the drain plug.

If that doesn’t work, then try the methods already mentioned.

I would try all sound suggestions before tearing down the engine.

The proof will be when you run it under load. If your oil pressure is higher and it will sustain a load longer than before, that’s progress. I would load it to just below the previous max and see what it does.