I have a feeling you'd have a hard time replacing that window with another like window - unless you happened to find one in an RV salvage yard. As far as small eggdom, I've seen them on 1983 and 1983 Scamps. They're not as common as the smaller, more horizontal single-pane Hehrs.
I would think you could replace the glass with new. As for the way it sits in the door, I would start by removing it. Best case scenario you can re-seal it when you put it back in (with butyl tape or other appropriate sealer on the back side of the flange). Worst case scenario is you've got issues with the core of the door being waterlogged. That's fixable but is a bigger job (search "Con Boler
door repair" for one thread on it). Basically, the doors are two fiberglass
skins with varying types of filler between them. The fact of the skins being separated (but bonded) is what gives the door - and its curve - the strength to stay firm. It's kind of like an I-beam that way.
I think once you have the window out you'll know more and can then investigate/decide how to proceed.
Edited to add: Nearly all new windows
have radiused (rounded) corners, so you'd probably need to do a bit of fiberglassing to install them (filling in the square corners). You can avoid that by going larger if
you can do that without running into problems with the curve of the door. But I think the older windows
are worth trying to repair; especially if you are not replacing them all with radiused windows