On mine the reason the teeth were badly worn was the protometal frame was bent (rear windows W123).
I bit the bullet and bought one new regulator and unbent the other one.
Extreme rear window repair
http://www.peachparts.com/shopforum/diesel-discussion/254722-extreme-rear-window-regulator-repair-123-a.html
At the time the Arc Welder I had was unsuited for welding the teeth. I could do that now as since that time I acquired a flux core welder.