Once those rivets are drilled out you will need new rivets and rivet gun to reinstall. I understand you have no access to the backside. That is how rivets are designed so no worries there.
Send me an address and I can mail you a plastic one I think I have in a box over here. Or can order a similar aluminum or galvanized bracket. Use that old piece you have to redrill the new bracket in the correct spots.
For bolts and such use stainless hardware and nylon washers between trailer frame and stainless hardware.