A S2 Master and Servo should be fine seeing as both the Cosworth and S2 calipers use a 60mm piston. It's the disc and pads which are larger on the Cosworth items.

FWIW I understand Sierra Cosworth items are the only ones with fit without modifying. I know with mine the clearance was super tight ; I milled 1mm off the disc in the end as they did occasionally catch. They did not drag the car to a stop but the disc did occasionally brush the caliper.