Two options for you which should help relieve your stress over not being able to find exact replacements. Both options are relatively easy to do and fairly affordable. Remember when working on Vintage trailers it is OK to think outside the box and use alternative repair methods.
You could clean it up to remove the debris and loose corrosion and rebuild the area with some appropriate Epoxy Putty Stick material. That will hold you for a while until you possibly come up with replacement framing. Just do a web search for Epoxy Putty Aluminum or head to your local hardware store as they might have Epoxy Putty for Aluminum. Amazing stuff, works great. Try to shape it when applying it so that you have a minimal amount of sanding to do as it does get quite hard when it has cured.
It does cure quickly so the prep work will take much longer than the actual repair job.
2nd option I though about making some custom screen frames which I have the ability to do but that was more work than what I decided for my fix.
I did not like the way my window screening was done on my Campster
as it could not be easily removed for cleaning the window glass. So I applied a thin strip of Velcro to the metal widow frame on the inside of my windows
and then cut some vinyl coated fiberglass screening to the needed shape and sewed the mating side of Velcro to the screens. I have sliders but it might be possible to do something similar on your windows
as well. Here is an image showing one of the new screens partially removed, there is a red tarp over the outside of the window to provide some contrast as the sun was washing out the photos. I did use grey colored Velcro on the aluminum surface so it blends in if I don't have the screens installed. But I used black on the black screen material.