Have you priced a new motor?
Typically if the motor has brushes you can replace the brushes yourself. I don't know what brand you have, but all I have here are Quatro units. The ones with brushes are easy to replace, and a pair of brushes runs about $30. If the motor is brushless (or if the motor is completely shot) then you need to replace the whole motor. For my units they are about $175 I believe, but also user-replaceable. I have no idea of anyone that refurbishes them! Good luck!