I've made replacement frames out of .080" styrene sheet, with added (minimal) detail copying the Gilbert frame. Easily done. Got to admit, those cars are VERY nice. The new Lionel ones were meant to replace them. They're nice, but there's something very cool about the original ones.
The warped frames aren't only warped, but have shrunk. That could adversely affect the body shell. That's why I decided to make my own.