If the system is otherwise OK... just down on refrigerant after many years I'd try to add the R12 first. I don't know what the current going rate is for a pound of R12, but I'll bet its a lot less than a conversion, done correctly, to newer refrigerant.

If you have significant problems that require completly emptying the system and replacing components, I'd go for the conversion.

I'm not an expert, but the simplest conversion requires draining and flushing the system to get all the old oil out, changing all O rings, changing the filter/dryer and changing the expansion valve. The original hoses, even though they may not be rated for the newer refrigerant should be OK.

The original condensor, in front of the radiator, may be marginal. For full performance a higher capacity condensor may be required (optional).

As I've learned, repairing/rebuilding AC systems isn't cheap so that's why trying a pound of R12 first may be a good idea.