Chrysler Imperial clubs have been working for years to get parts suppliers lined up.
I bet there's someone who could overhaul the motor itself. The motor is rotor, stator, brushes, bushings or bearings, screws. Not much to go wrong.
You can get them on Rockauto for back as far as 1969. Is it possible that later ones would work with some tweaks?
Ebay -=http://www.ebay.com/itm/1965-74-Chrysler-Plymouth-Left-Hand-Power-Window-Motor-Good-Working-Original-/201887402001?hash=item2f016d4811:g:OTgAAOSwhQhY6~iG&vxp=mtr
R.