Just on improvements - and this may be an extremely dumb question - if you replaced the compressor with a R 134a compatible rotary , could you still run the system on R 12 A refrigerant and not replace the expansion valve in the air box ???
The R 134a compatible rotary has a higher rated pressure but the rest of the system would remain exactly the same - unchanged and in fact the system pressures , with R 12 a in it would be unchanged ??
I am now waiting for the slings and arrows of experience to arrive ...!!!