I've had luck with the following non oem options:
Replace motor brushes rather than buy new unit. (Sounds like you PeterT)
Replace oem control with home-made speed control (nice, but not really worth the effort.)
Use relays or transistors to ease load on ailing oem speed controller, or maybe wire relays/tranz to activate via A/C and heater.
Bypass oem switch completely with simple toggle switch (easiest, but no speed control).

Just some ideas.........