Well, in the 6800 owner's manual it says to not operate the machine on a slope greater than 10 degrees, but I think that is more of a safety-rollover thing than for the engine. I believe the engine is a pressurized one, so it shouldn't matter. May have just been its time to go...Not sure if there's a repower kit for these.
Might want to check with Small Engine Warehouse, or any others out there, or look for a rebuild kit if available.
Rattosh51
YT6800