I looked at the picture.
Let me weld it before you try anything else. I've removed hundreds of broken screws and bolts like that, some much harder to get to than that. Of those hundreds only a couple didn't cooperate and needed actual machine work, drilling and Helicoil or what have you.
Worst case is drill it out and install a helicoil. I like that better than changing screw size in most cases.
