Many Fiesta ST suffer from engine failures nowadays. Best would be to buy a whole ST with a dead engine and rebuild it.

The Duratec-HE won't fit to the "old" Ford gearbox bellhousing stud pattern, so you'll need a Duratec-HE gearbox PLUS Ford shortened the bellhousing to make the engine fit into the MK6 engine bay. These gearboxes were just used in the MK6 ST. Gearbox and engine mounts are MK6 ST only and electrics/wiring on such a modern car will be a pain in the arse.