[Ron61]

As Sal said above, if you can get to the side of the frame the nut was welded to, I am guessing it was welded, and cut a small access hole, I would try to do it where the nut was welded. Then make the hole just big enough to get the nut out through it. Then take a piece of plate that is strong and tack the nut to it and get it lined up so your bolt will still go into the nut. Once you have this all straight, weld the plate to the frame with the nut now welded to the plate on the inside. I am sure there are simpler ways to do this and depending on which side of the frame your nut was fastened will play into it. Good Luck.

Ron