I'm not sure I would worry too much about corrosion.
It's going to be difficult to get that screen in full-size. You don't need it to be full size, you just need enough of a backer to keep the filler from sagging too much. And you can re-apply if there are gaps.
If the frame were worth fixing correctly, I think the tube can be replaced without killing the lug chrome. Gugie has shown the way with replacing fork steerers. That would be my approach