Assuming that she still structurally solid, I would first try to deactivate the rust that is present, maybe sand some of it off, and then repaint the frame. I use summit racing enamel chassis paint and it's worked well so far!

Note that there are a couple of companies out there that make internal spray paint cans for chassis and frames, basically a rattle can with a long tube with a sprayer at the end. I used that combined with a broom handle with some rags soaked in alcohol to fix a bit of rust in that "tube" above the rear axle.

Alcohol soaked rags to clean out the area, pushed gunk out. Then I stopped one side full of a bit of towel and ran the tube back and forth spraying very generously. I can't remember the brand, but there's one that has black and then like this neon kind of pastel green.

Anyway, maybe see if there's some kind of rust neutralizer that you could spray inside of the frame just to make sure it's good. Although I would make sure it was totally dry before applying paint inside