i think you might be able to fill it if you take out the speed sensor, i can't remember exactly but i'm pretty sure it's not on the underside, if it's lateral it might work, only problem is you don't know what the level is.
Jeffy's suggestion with the notch is probably best, just a tiny bit and then use a punch to move it, i took off many rounded bolts like that (usually just use a chisel to put a dent in it and then with a punch just tap it)
i wouldn't drill, no matter how much you try there will still be shavings going inside.