I usually buy a few anyway if I have to do a job like this. Also, I grind the end of the broken bolt so that it's a good flat surface and it's easier to center a pilot hole. They don't usually break off nice and flat. Is there any way to weld a nut to what's left of the bolts? If you use one whose inside diameter is a little smaller than the bolt diameter, you might be able to apply enough weld material down into the center of the nut to get it to work.
