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Aaron, first, you have to define a lot of rust. Is it like swiss cheese throughout, or is it solid overalll with one bad area?
New 64 frames are available as are sections, crossmembers and channels. You can even go tubular, or buy a later C3 frame and retrofit it for your 64. Lots of options.
Post this in the C1/C2 section and you will get a lot of advice. Just be prepared with PICS!
Depending on how much and how severe the rust is, would depend on what you do. Frames can be fixed if not too severe, I am assuming we are talking more than just surface rust. Perhaps a few pics posted, would help in deciding how bad it really is.