Yes, we had the same problem with our original 67 M52. Ammonia smell means that the piping in the back is leaking. In our case, it was also very obvious where the leak was, too. And same problem you have: Finding a person to redo the piping is like finding a needle in a haystack.
In the end, we wound up getting a new fridge ($$$!!!). It's a Euro unit from Dometic, smaller than the full-size units, but larger than the smaller unit that they market in the US. But we'd still prefer the old M52!