I would never use a junkyard compressor, you're just asking for trouble (been there - done that - learned my lesson! not worth swapping three out to get one good one). The prices on new compressors have come down, so I always go that route. Even "remanufactured" compressors I have found to be junk, and either be noisy or leak or go out all together six months later.
There's nothing wrong with using factory components designed for R12, and retrofitting them for R134 so long as it's done properly. That means change the filter-drier, new expansion valve or orifice tube, and completely flush the entire system with approved AC flush solvent, and evacuate and recharge after installing R134 charge port adapters and adding synthetic oil. If there is any trace or R12 or mineral oil in the system, it will mix with the R134 and synthetic oil and create acid that will destroy the new compressor.