Sounds kinda like some damage to the rings, or thereabouts.
You’d need to disassemble the motor down to the components; piston, rings, cylinder wall, to see how much of each was damaged. In other words,
a
lot
of
work.
Perhaps you can find a donor assembly somewhere, which died only because someone didn’t understand carburetors and gasoline, and let it die because of that rather than extensive use.
Frnkenmotors are always a good solution anyway.
Agree that it seems like something internal. It’s a Briggs OHV motor so popping the head off and taking a gander at the cylinder/piston for issues and then formulating a more informed action plan seems right now to be the most prudent. Like after mowing season when I can disassemble it and have it unusable for however long it takes.
Of course, since I intend to continue lopping the top off of the advanced grass substitute using the Little Red Ridingthing until that time, if there is really something serious I might be forced to act sooner than I want.
But then I’ll have more knowledge of the situation if that should occur. ;-)