I would cut some 14 to 12 gauge sheet metal into about 5 inch strips and tack weld them to the fender in place where the mounting flange is gone or in sad shape. I would not use adhesive. You have a MIG and the welds will be on the underside of the fender so as long as they have decent penetration they should work fine and to heck with their appearance. If you are using flux core wire be very careful not to blow through the metal. If using MIG wire and gas I would start with a amp setting of B and a wire speed in the 11-12 range. Just use tack welds. If you want you can always come behind it and fill any gaps with epoxy or JB weld.
Good luck.