First off , if that refrigerator is still working; you need to take steps to protect it AND your house. That wiring, at least the exposed stuff, is dangerously close to a short circuit condition. The insulation is just too rotten to deal with, and if I was there; I would totally rewire the fridge for you. If the compressor start components still work, and it seems like they do, you're golden and leave well enough alone. The compressor doesn't look bad itself on the terminations, maybe just rewire to the compressor leads. Those extra leads in the picture can be anything. A thermostat, a mullion heater, or a light in the door could be part of the circuit.
You do not want the compressor to run forever, it needs to cycle. Freezers especially were forced to take at least a 15 minute break every 6 to 8 hours through use of a timer. You won't see the timers in 1950s models so much , but as the use and lifestyle changed, they were very important by the 1970s.
R12 is still available, legally. Practically: no one carries it or much at all. The cost is so high for a jug, and the demand so low that many shop owners don't see a short term ROI on having the stuff, plus some places have a business inventory tax so there's that. What I'm saying is R12 is rare, but not impossible. The next is service. A hermetic system such as yours was routinely serviced and charged years ago. It was my day job for 20 years and we had tools for such. It's not hard, but there is a technique. I do not know the field today, and I don't know what today's techs know. You can weigh the charge, or use a frost line.
Randall's gasket idea may work for you. I don't think so much for freezers, but fridges may work okay. You can check your gasket one of two ways. My favorite is the dollar bill test. Put a dollar on the jamb and close the door. A good seal will give slight resistance when you pull the dollar out. No resistance usually means no contact or a gap. Another is a chalk test. Take some blue carpenter's chalk and coat the gasket. Shut the door, then open it. Wherever the gasket contacts the jamb, there will be a blue line or mark.