Furnace compatible with 1963 openings

[Aktundra]

I posted this same question in the propane forum, but this looks like a better sub forum, I’ll try to delete the other posting.

My 1963 Bambi spent a lot of time in Florida. Presumably while there, the furnace was removed and a 110v household AC unit was bolted into the front window. Does anybody know if there is a replacement furnace that mates up with the original opening ? Will anything do the trick & just installed with ducting ? Probably the same question for a replacement water heater ! (Sorry, can’t figure out how to keep the iPhone from posting pic’s sideways!)