RC4RC, I honestly think you'd be better off buying new 50MHz receivers. Rebuilding your old 72MHz RXs don't make economic sense unless they are $150 PCM 9 or 10ch ones. You can buy 50MHz 7ch. FM RX for about $60 brand new.
I fly 50MHz but have a sizeable vintage collection of 72MHz RX as well, a couple of which are practically brand new - all on various frequencies and different manufacturers too. They work great in electrics, park flyers, and gliders, which I don't typically fly at a field and so air traffic is much less of a problem, and dare I say been shot down is somewhat less of a PITA.
I keep an Eclipse w/ Spectra just for flying them. This sucker flies everything FM under the sun.
Just an idea.