Sounds like you need a refrigeration specialist to have a look. Refrigerant from 1974 (freon?) is probably not permitted or available. Newer refrigerants may or may not be compatible with your setup because of piping or seal materials. The specialist may be able to help you out with what you have and get it to work. He may also suggest replacing the entire mess with a more efficient modern unit that might cost less to run and end up saving you money.