I had one of these on my last boat driving an older whitlock system with torque tubes and bevel gears.

I never had a problem with mine but I did meet another cruiser in the Caribbean who had the same drive on a much larger boat;
He had to replace the friction material on the clutch and was able to disassemble the unit himself.
Unfortunately, I don't know how he did it, only that he was able to and eventually got it back together and working again.
His boat was a fair size wooden schooner out of Rocport Maine named Kookla (his name was Ted).

If you have the time, I would try to find a good electric motor shop to look at it.

There may just be a sticky gasket or some other goop under the cover ? You could try heating it up with a heat gun.

Cheers,
JM.