I am not sure where in the U.S. it is legal to stamp a frame. It may very well be legal, but how do you ask without raising eye brows? I do know after being in the old car hobby for many years and I do not know if it is 100% legal, but many bad car frames, get the original stamped number area transferred over to the good frame. Done properly, it looks proper and that is how I would do it. Since I am also in the car hobby, the ser. no. are everything, to the originality of the vehicle. If I were unable to repair a bad frame, I would find that good frame and transfer what I needed over to make it right, appear right and the paper work would match all the metal work. Again, this is how I would do it, your mileage may vary.