This whole thing looks bad. ALTERNATEL1: I would take a 1/8" Ø drill and drill out all the rivets around the plug carrier and that one that isn't part of the carrier. Remove the carrier and put it in a drill press and drill out the "bolt" with an appropriate size drill. The threads should be good in the aluminum carrier from how it looks to me so be careful drilling it, then use a step drill or a tapered reamer to expand the hole until you will see the threads. Then clean them out carefully with a steel tooth pick. ALTERNATEL2: you should determine the material the screw is made from using a magnet, if it sticks it's steel and if it is then find a heavy washer that will fit inside the lips of the carrier and arc weld it to the screw, then weld a nut to the washer and then use a ratchet wrench and socket to try to remove it, cool it down with water after you weld the nut on. ALTERNATEL3: heat the hell out of it with a propane or brazing torch and try a proper fitting screwdriver to the screw and a 12 inch adjustable wrench on the blade end next to the screw to try to turn it out counter clockwise. After you remove the screw from the carrier and get it cleaned up to receive a new plastic live well plug, which I believe this was designed for, then you put it back in place, held with a couple 1/8"Ø pop rivets or sheet metal screws to hold it in the same place it came out of and then re-drill all the holes with a 3/16"Ø drill bit and use swage style Aluminum rivets to hold the carrier back in place. A bit of 3M 4200 would be helpful on the mating surfaces.
This look like a big job but with the proper tools and a bit of effort it can be put back in original condition.
"It's not the game but the chase ~ not the trophy but the race !" from my Dad, many years ago.