Try the impact driver, press down really hard and give it a firm tap, you could try a sharp punch and drive the screws round with that, or drill, I think the screws are M4, drill a shallow pilot hole 2-3mm (take care not to break the bit!) then use a 4mm bit,(don't break it!) this should take the head of the screw off, lift the top off and use pliers or grips to take the screw out of the master cylinder.
Replace them with "allen" screws
Impact driver will be a good investment, as is a hot air gun
Anything can happen in the next half hour