Only two concerns in general. Getting the door seals to fit tight and a place to mount the hinges. The hinge mounts are usually just bolted or screwed on, so its a matter of finding a place solid enough to either tap threads or accessible enough to use nuts and bolts. Seals (can you get repalcement seals for the old door?) are usually simple pressure fit, so as long as the existing sels meet up with a flat surface you should be OK. Check the dimemnsions on the various size fridges currently available or tae your tape measure to the salvage yard if looking at used ones.
I wonder if the cooling unit in the old one can be replaced? The technology for gas refirgerators didn't change much at all until several years ago (with the addition of a lot of electronics), so the basic unit might be replaceable with a more modern one, even though of a different brand. Replacements aren't cheap, though.