I would make a metal piece of the same shape and design as the plastic bracket with a hole in where the plastic has a fixing hole. then pop rivet or use tiny screws and nuts to fix it to the broken plastic piece. Rivets from the top, through the plastic into the metal piece where it locks in. I would use 1-2mm aluminium sheet, you can buy that from Bunnings for few dollars.
I would place the aluminium underneath rather than the top of the plastic bracket so that it remains tight against the mud guard. otherwise you get a gap between them.