[hop up]

Great ideas by members but if you do chose to drill it out . I would first get the surface flat and locate center of the bolt. I would then center punch the bolt right in the center. Then drill a small hole to the length of the bolt and drill a bit larger hole. As you drill closer to the size of the bolt diameter you may loosen up the bolt to unscrew it . You can also use a center punch to push in the thread sides and get some needle nose pliers in there to unscrew the left overs. Now worse case is you drill to appropriate size 1/4-20 would be #7 drill etc and tap it out.
I would also recommend Kroil its awesome stuff and truly a rust buster