I suggest it's well worth trying to get this system working again with R134a. First you need to get it presure-tested in case there are leaks. After all, you don't want to waste expensive R134a refrigerant. You should also check that the flaps, blowers and everything are working, if possible, so you're not wasting your time and money. Finally, a change of condenser to a parallel flow type would not go amiss. R134a should then cool better than R12 ever did.
I suggest the Harrison A6 compressor (if that's what you have) is well able to work with R134a, as long as you drain the compressor and the whole system of every last drop of mineral oil and then refill it with the precise required quantity of Ester (not PAG) oil. The A6 is a real work horse - an excellent compressor - and has an oil sump.
This guidance is not comprehensive but it's a start. You also need to change the receiver-drier to an R134 type (it should be changed every so often, anyway) and be aware of numerous other points. But it's relatively straightforward. For further info I strongly recommend the air con forum at
I'd never contemplate using any of the so-called drop-in refrigerants like the one you mentioned. R134a should work much better.
Regards George