It sounds like you are facing a much bigger problem than merely replacing the compressor. Upon compressor failure, debris is strewn throughout the system. When replacing the compressor, you MUST THOROUGHLY flush the ENTIRE system, replace the filter drier and preferably add a suction side filter in the line before the entrance to the compressor. You will then need to evacuate, and charge.

R12 will be a small fraction of the cost of this job, so I would STRONGLY recommend that you stay with R12.

Best of luck,