I rebuilt my heater and replaced the original 6 volt motor with a 12 volt motor that I ordered from Grainger online
http://www.grainger.com/Grainger/wwg/start.shtml . You may not be able to buy directly from them like I did, but you can determine what motor to get there and pick it up at any electrical supply store locally. I think I paid about $20 bucks for mine, and it fits and runs just fine.
If you decide to rebuild it, make sure to have the heater core pressure tested and repaired if it leaks.
