Okay, that's what everyone's saying, but I'm not convinced that's the way to go; or at least, I'd call that Plan B. For Plan A, here's what I'd do:

Remove and/or neutralize the rust chemically. Fashion a new cable guide from thin steel or brass. It won't be perfect, but make it as good as possible, using riffler files or whatever it takes. Epoxy that onto what's left of the old guide. After the epoxy has completely set, go back over it with the files again, clean up the glue, make the glued joins look as good as possible. Make sure you can fit cable housing through the hole! Now paint the guide the same color as the frame.

If this doesn't work, you can still go to Plan B.

If Plan A and Plan B don't work, you can always got to Plan C, complete restoration.