If going at it from the inside doesn't work you have several other options. You look like you have a nice paint job so I would suggest some protection with all the following suggestions. Cardboard, pop, cereal, beer box etc with painters tape holding it in place should work fine. You may or may not have any of the tools/equipment to do any or all of the suggestions??
A punch and hammer working in a CCW direction trying to back out the bolt until you can get something on it. Don't get to close to the edge.
Welding a nut on.
It looks like maybe someone tried center punching the bolt?? If so is it centered? If not use a rotary tool to get a flat area to center punch and getting the area as flat as possible. Center punch and Drilling the hole directly in the center and straight are important. If you can eye ball the drill and keep it straight good for you. If you are unsure about keeping the bit going straight get a 1/4" socket, preferably a deep one that will fit in the recessed area on the bolt. It's important that the socket fits flat and not tippy. Put the drive in towards the broken bolt, put your drill bit through into your center punch mark and space the drill bit equally around the sides of the socket, this should help keep you centered with a visual reference. Now drill your hole watching that the bit stays centered. If needed you can have the drive side of the socket facing the drill to get a tight fit with the recessed hole. Drill the hole to the correct size of your easy out. I prefer the straight ones since the tapered ones can wedge the broken bolt in.
Hopefully you can/have get it off on the inside.
