Well, there are some rust converters, like Por15, Ospho, and others, but they essentially just kill the rust, but in no way "strengthen" the metal. Frames are generally pretty thick, and you will have to assess how much the thickness has been compromised by the rusting that has occurred, i.e, is it just cosmetic, or structural damage at this point.

I would have it sandblasted to see what you really have left.

Don