Brake reservoir screw head issue

[max]

1 Would try to remove the one good screw and see if you can rotate the lid so as to break the head loose.
2 sharpen a flat head screw driver so it has the widest grip on the bad screw and very sharp biting edges.
3 try tightening and un-tightening alternatively and use some wd40 or equivalent.

Thanks for the picture, that really helps.
Also put the new screws in with some grease in the threads, let us know how you get on.