With a torch, Vise Grips, a small scissors jack, and some JB Weld, you should be able to get it just about perfect again, but it will take a little work. Ideally you'd have two torches, but I don't think that's really necessary.
After removing the seat & fender (duh) and cleaning the paint off the bends (I assume these are right after the tube gussets from the rear seat mounts?), try setting up the jack between the center of the loop and the frame crosspiece between the shock mounts, pushing back on the loop. Then heat the bends red-hot until the loop starts to move, squeezing the bends together left-to-right with the Vise Grips. The idea is to pop the tops of the bends up using simultaneous squeezing/stretching action. The hotter you get the bends, the better this will work.
Once the tubes are sort-of round, you can try heating them again and pushing straight down on the loop to get it level. When you're happy with the loop's position, file/JB Weld/sand the bends to hide the metal deformation, and you're ready for paint.
If the bends are very sharp and the tubes are straight on either side of them, you might also consider cutting out the bends and rewelding the tubes. The loop will be slightly shorter, but it may not be noticeable.
If the above doesn't work well enough or seems like too much work, you might try contacting some of the vintage parts places on eBay and asking about frames, which I believe they have trouble getting rid of. They may be able to sell you just the rear loop, or - if you're a glutton for punishment - you can replace your frame.
Ray