The factory bolts are "button head socket cap screws" - really hard if not near impossible to drill. Remove them, take them to your local hardware-type place and buy a couple "socket head cap screws" same size/thread/length.
Try to get the exact length if you can or go a bit longer and grind/cut them to length. Too long and bolts will hit brake rotor. Too short and your brake caliper may fall off. A little long is preferable to too short. Don't remember exactly what they are, probably M8 or M10. Get a couple of nuts too. Thread the nuts on your bolt (tightly) and use them to clamp the bolt in a vice or vice-grips so the bolt doesn't try to rotate while you're drilling it. Drill the bolt head so that the hole goes through the socket flats. Clean up the burrs in the socket so you can get your wrench in there later. If you need to shorten the bolts, thread a nut on first - it'll help clean up the threads after. You may go through a couple of bits and bolts before you get two good ones. You end up with something like this:
Safety wire bolts are common in the aircraft industry, so you can sometimes find them under 'aircraft parts' or 'aircraft fasteners', but for me it was just easier to make my own. Don't forget Loctite.