Oh, yeeouch! There's always the local welding shop. They can weld, add bracing metal or both. Or a new frame.
Paul had our frame taken to Machinists, Inc. where he used to work and they did a scan of some kind and welded and added where they said it needed it, then Paul scrubbed down the rust, primed and painted it there with white industrial metal
paint. They helped us a lot with Peanut. It took several weeks.
Maybe call some weldshops. Some of them can make new frames as well as repair old ones. Gonna cost you.
We got a crane operator from Mach. Inc. to lift Peanut off the frame.
Old Peanut coming off the frame.
Repaired Peanut showing white tongue (white so I don't fail to see it and trip).
BEST to you; this is a vital job. We also found 4 out of 6 bolts holding shell to frame were rusted through; the other two were half gone.