man that sucks, the only ting i can think of is if you make some kind of jig to hold the bit centered.

maybe trace the back of the pumps bolt holes and drill the holes out of a thick piece of flat stock, bolt the good bolt through the jig and line the other hole up with the broken bolt and try to drill it out.

come to think of it, if you pull the pumps pulley off you could maybe use the pump its self to line up the drill bit.