If there is any bits of the cross head left, just use an impact driver. As far as I know they are just there to stop the brake disc falling off when you take the wheel and calliper off.
Just replace it with a new one, and don't do it up too tight!
It could fractionally affect the balance of the disc and therefore the wheel if you don't put it back.
Best of luck!
What type and size fit it?Besides too much torque, most of those retaining screws are ruined because they are mistaken for Phillips. On the head, you will note 4 markings in addition to the main cross which means that it's a Pozidriv screw.
What type and size fit it?