That's kind of a generalization, when the R12 system on a 20 year old car goes south, most people opt to update/convert it to an R134 system rather try to maintain the old. There are lots of great threads on this forum regarding what needs to be done for a conversion and whether one would want undertake it themselves or have it done.
Lot's of threads about the pros and cons of keeping the old system intact and using alternative refrigerants. There's too many details to cover it here and I don't feel qualified to elaborate anyway.

Same with the head gasket, while I feel qualified to discuss that, I already have in previous threads as well as many others here. Best to just search for them.

Do a Forum Search on "head gasket" (M103,W124), select "search only titles"

Similarly, search: "Air Condition"