Brake reservoir screw head issue

[pekingduck]

Here is how what I would recommend.

Do not get another master cylinder. Order a new cap, diaphragm, and the spacer plate in between.
Use a hacksaw to cut across the top of the cap, and into part of the screw head. If you take any care at all, you won't even be cutting into the rubber.

Lift the large piece off, and make a small cut as shown. Now, with a small pair of pliers, the remaining pieces will just break off. The screw will be exposed so it can be twisted out, and the fluid will not have been exposed to air or contaminated.