Wow that really stinks!
If the frame is rotted, maybe this is a great time to start looking around for a replacement. For $3000, I just picked up a T-top Formula that is in overall terrific condition with not a single spot of rust on the frame. http://www.fiero.nl/forum/Forum1/HTML/080989.html
Locally here in Buffalo, there have been a number if interesting Fieros in good condition for sale as of late. If you have the cash I am sure you can find something.
BTW, do you know who hit the car? Can you turn it into their insurance? About 10 years ago, I had someone hit and total an 87 4 cyl coupe with 130K miles, a terribly rusted frame, and a motor that burned oil. Turned it into his insurance, and talked them up to $2000 for "repairs." With that money, I found an 85 SE V6 with 67K miles and a rust free chassis! It ended up being like a free trade in/upgrade (the hard way!).