* Engine Bay Size: The 1.2 engine bay is significantly smaller than that designed for the ST's larger and more powerful engine. You'd need to drastically alter the chassis to accommodate it.
* Subframes and Mounts: The ST's engine and transmission mounts are different, and the subframe likely won't be compatible. You'd need custom fabrication.
* Wiring Harness and ECU: The wiring harness and ECU are completely different. Integrating the ST's system into the Mk6's would be a massive undertaking, requiring significant electrical knowledge and potentially custom programming.
* Transmission: The ST's transmission is different and likely won't bolt up to the Mk6's differential or driveshafts.
* Cooling System: The ST engine generates significantly more heat. The Mk6's cooling system would be inadequate. You'd need a larger radiator, different hoses, and possibly even a different fan setup.
* Brakes: The increased power demands of the ST engine require more powerful brakes. The Mk6's braking system would be insufficient.
* Exhaust System: The ST's exhaust system is designed for its engine and would need major modification or replacement.
In short, while theoretically possible with significant engineering expertise, fabrication skills, and a substantial budget, it's overwhelmingly impractical to swap an ST engine into a base Mk6 Fiesta. The cost and effort would far outweigh the potential benefits. It's far more sensible to consider other performance upgrades that are designed to work within the limitations of the original 1.2 engine.
