Good luck with that one. How do the windings look? Blue? They can overheat and develope shorts from the broken down insulation. Hows the brushes and commutator? If you overheat a motor, there's probably more that can go wrong than just weakening the magnet.
Rubbing, I doubt it. I read about magnet zapping, a device that creates a strong field to restore a magnet, but I don't know if you can fix one that has been overheated. Check out the windings, brush and commutator before you consider spending any more time on this motor.