I remember doing this years ago on dad's 77 model. If the gears are gone I couldn't imagine that being repairable. I'd check the gears are meshing in line properly, as they're pretty thin. Replacements are not hard to get hold of.
To replace take the inner door panel/s off, tape up the window (use lots of tape, they're quite heavy), and the rest is pretty much self explanatory. The unit comes out through the holes in the door (with a bit of wiggling, and scun knuckles).