Convert the system to R134 refrigerant along with a Sanden style compressor and a larger condenser. If it were mine, that's what I would do. Those older dealer installed AC units are a ton better than some of the stuff on the market today.
Mine used a (think Ford like) York compressor. Doubt you could get parts for them now. Plus a Sanden is smaller. OP some of the outfits offer a bolt on Sanden conversion or IE just buy that part of the system they sell