You really need to put some pressure on it and then heat the area around the rivit so it will reshape. If you have access to a "friction jack" or a Porta-power that would be helpfull. But if you don't, you can use an old bumper jack. Position the jack against the base of the rear rb bracket. Then at the other end find a way to attach it to the outer most of the bent one. You can put a bolt through the bracket and jack up against it. Or you could bolt a small chain to it that is looped back to the bolt. Put the other end of the jack into the loop. Jack it out to apply pressure and heat the bent part at the rivit. You will probably need to "help" it with some hammer work but it will go. Then weld the rivit to the bracket around the hole.
If you are doing a resto that you want nicer than that, drill out the rivit first. Then after you have the bracket hammered nice and flat on the frame, re-rivit it.
