As said the Zetec S is a straight swap - but for ST bumpers you'll ideally need ST front wings and sideskirts too. Both cars also had different grilles although some Zetec Ss left the factory with the standard grilles, they're too short, and don't look as good.
This a ZS with the wrong grille, a ZS with the right grille, and an ST grille. All will technically fit, but not all look right.