Looks like most of it is in decent enough shape from the small picture and if it's hidden, I would just stabilize it and move on. If it is too thin or holey for comfort, I attack areas like that a little bit at a time. Try and cut out small areas at a time and fab the replacement to stitch weld back in if you can't source a full or partial replacement part. Again, if it is hidden under an interior panel get it close. The final shape isn't that important unless you are OCD or doing a high end concours rebuild. I understand wanting it to be all original but if it's a support panel or something hidden under headliner or trim, I move on to bigger fish.