looks like you have aluminum framing, and the walls seem to be in ok shape. That's all good news! I'd definitely replace the foam sandwich you pull out with plywood just for ease of construction. Only downside is weight, but doubt it will be that much more really. And plywood will be a lot easier to work with.
I had good luck with
The Rot Doctor when I did my rebuild. I used it not only to treat damaged wood that I could not remove and replace, but also painted it on new plywood that I installed to make it waterproof for future leaks.
on edit: I see this was a few months ago. Hope the repair went well for you!
