R12 is still out there, a shop should be able to get the real stuff. That is what is in mine. the replacements are often some kind of hydrocarbon (propane/butane) from the research I have done. IF you cant get R12 I would suggest R134 and a replacement Condenser like a parallel flow and maybe even a aux fan. New drier and complete flush of the evap, an a new compressor just to be sure, replace everything (evap too if you want to pop for a new one). Oh and barrier hoses as well. With all new stuff you will be into the price range of a vintage air, but will not have to do the fab work, and it will be original as far as the controls. I like the OE style since it maintains the correct use of the outside air chimney for ventilation.
The correct way to diagnose an existing unit would be to have a shop evacuate the existing gas (if any) then fill with nitrogen to about 80psi (the normal off pressure for todays heat) see if it holds pressure, use soapy water to find leaks esp around the condenser. Look at the connections as well. The evap lives in a safe place so it will prob be ok. if pressure test checks out, before pulling a vacuum check to make sure the little line filter at the outside firewall connection of the high side is not plugged up, Flush the condenser with cleaning fluid install a new drier, pull a vacuum. If it hold near 30inmg for several hours after stopping the pump, then they put in the correct amount of R12 (assuming no oil was lost). The system is very simple, and as demonstrated in the video works just fine. Mine had a slow leak at the condenser, I was able to get a nice NOS unit to replace it as well as a now hard to get 33286 drier. This meant I was able to get it completely original. If I could not get the condenser and drier I would prob opt for a after market condenser of the same approx. size and modify the inlet side hard line so I can use the easier to find driers with inlets and outlets at the top.