Hi!
An accomplished R/C pilot can see at once if the engine is worn out by just turning the engine over....! Simple as that!!!
It's the piston to cylinder fit that is important. If the it's worn out... just get a new piston/cylinder set!
But if that guy knows anything about engines I assume he has tried a "Uni-flow" tank set-up ...or a Tetrra "Bubbleless tank?
...And get rid of that MA prop! It's the worst prop you can use. Get a RAM or APC !!