Honestly, I've seen worse. I don't believe there are any replacement pieces for that, but you might check with @moparmark on this site too see if he has a patch panel. If it doesn't look to terribly bad up close, you "could" wire brush, weld in a 1/4" steel plate & rustproof what you have? Maybe a frame shop can take a look at it?
Look a little closer at the 3rd picture. It "looks" like a crack has developed. How bad the rust is, I can't say.That looks pretty solid to me, I certainly don't see any structural problems.
Are you just worried about the cosmetic looks?