It sounds, at this point, like the starter motor is bad, or current to the motor is being limited by an as-yet undiscovered resistive connection. However, I believe you said you connected a battery directly to the starter motor and it could not crank well, which would eliminate all electrical connections and the starter relay. From the exercises you have performed on the engine, it does not appear that the engine, transmission, or clutch has a mechanical problem.Update: remounted the cleaned starter motor. The engine turns over without any suspicious noise or sign if the spark plugs are out. Still somehow it feels to me slower than usual (I'm not sure about that, it's just a feeling). If I put back one plug, it sometimes fails to turn, if both plugs are in, it fails almost every time, a few times it gets one or two revolutions, and then gets stuck at the next compression stroke. I checked and cleaned the connections, I tried to feed the starter motor directly from the battery with jump cables, same result. Any ideas?
It’s always a tense moment when you try the new part. Swapping the starter motor with a new one may lead to a celebration, or “oh crap, now what do I do.”