Tiny has the right idea. If you have access to a band saw cut a one foot length of hard wood or even very dry 4 x 4 treated post lumber and make a wooden buck the contour shape of the concave side(inside) of the bumper. A hydraulic press would be the best choice to reshape it. The pressure will stretch the metal some, but you can enlongate the mounting holes if necessary for fit.
It won't pass the 100 point judging, but I don't think that matters to you at the moment. grin

Drew