Stephan, I saw your request on MMD.  Several questions come to mind.
These motors are very reliable and seldom need anything more than new
brushes or new brush housings.  The motor may APPEAR weak if there is a
poor electrical connection in the system.  One such place is on the
resistor mounted on the piano rim.  If the screw posts are loose on the
resistor, the motor appears weak.

I suggest that you by-pass all control wiring and hook the motor directly
to 110v.  This will prove if the motor itself needs service.  The brush
housings inside the motor are made of pot metal.  If they have swollen,
it can restrict the brushes from making good contact.  Robert Streicher
makes new Brush housings.  Sometimes it is possible to file the brushes
down ( on the sides) to make them smaller, thus allowing free movement
within the brush housing.

One last area to look, is the slip clutch was comes into action on
rewind.  If this is out of adjustment, the roll will not rewind without
some assistance.  Observe when the rewind stalls if the motor is still
turning.  If so, the clutch is too loose.

A replacement motor will cost $500, if you can find someone willing to
sell.  I've heard that there is a small motor shop in Cleveland that
MIGHT rebuild this motor.

Bob Taylor