Stuck Master Cylinder Screw

So I was replacing my front pads on my '82 KZ 440 LTD today, and after getting 'em in I was going to bleed my brake fluid but....I was not able to get one of the two phillips head screws out. Since the screw head is flush with the top of the MC using vice grips, or a hacksaw are not viable options. I've read in other posts to try and use an impact driver, but I am wondering if it is possable to damage the MC with that kind of force, and should I remove the MC from the bike first?

Thanks

Those bolts were likely chromed originally and rusty. Use the hand impact driver with the mc in place. Won't hurt it as you aren't banging that hard. Make double sure you use the right bit. If you do mess it up and the head breaks off as it likely will, don't worry, when the cover comes off, there will be exposed bolt left you can turn out with a pair of vise grips. If that fails, it is drill and tap time.

I am going to move this to chassis section... brakes are not in engine section. Cheers!

So what tpye of screw would make a good replacement? The MC looks pretty rough, so the appearance of the screw isn't too important (at least for right now).