Will fiesta mk5 zetec s disc brakes fit a 1.3 mk5?

No, Fiesta Mk5 Zetec S disc brakes won't be a direct fit on a 1.3 Mk5 Fiesta. The 1.3 likely has smaller diameter brakes and different caliper mounting points. While you *might* be able to adapt them, it would require significant modification and likely custom fabrication of brackets, and might not be road legal or safe. It's far simpler and safer to use brakes designed for the 1.3 engine.