- If the rotor is really so expensive, is there another that fits?
Automotive?
Maybe, but don't worry about replacing the rotor now if it's running
well.

- What gas/oil mixture does this motor run?
You ran it already without knowing? - Be sure to find out and use the
right mixture before running it again or you'll destroy it.
And don't run it without muffs to get water to the engine or you'll
overheat it for sure.

- What kind of oil should I put in the gearcase?
90 weight gear oil...any marine dealer or auto parts store.

- How do I check the waterpump (or impeller)?
Get a manual and replace it. It will need it.

- What compression should this motor have?
Don't worry about the compression if it runs good.

- Is this motor really garbage? Ok it has no power trim/tilt .. but
aside from that?
Not if it runs good.

- how does one decarbonize OB motors?
Don't worry about it if it runs good. Why would you think it needs to
be decarbonized?

John C.