Well, if shorting the wires on the foot controller didn't make it run full bore, then it's not the controller. Did the motor rotate freely when you had it out?
The motor runs so there''s evidently not a short or open (though there could still be a high resistance) and you've cleaned the commutator and checked that the brushes are intact.
My next move would be drop the motor out to disengage the drive gears and see how the machine spins over by hand.
Maybe disengaging the handwheel like you're going to wind a bobbin would tell you something. If the motor is still sluggish, then it's something in the motor or electrics. You've got resistance somewhere, either electrical or mechanical.
It;s an interesting problem, since it's not mine ...
