Okay, telling me to sell my car and buy another one is not helpful. That is obviously not a reasonable option if I want more power.
Without going into specifics, Scotman gave you viable information. Many would agree it to be the easiest route.
Here are some specifics off the top of my head...
-The ST 1.6 turbo is Direct Injected and the regular 1.6 has Multi-port Injection. Cylinder heads will be different casting or machined based on application.
-without the DI, the ST computer will not be able to operate the engine under the same parameters...it will never run right. This is probably the magic bullet.
-The ST 1'6 turbo is factory turboed therefore would have a turbo oil return boss machined. The regular 1.6 wouldnt, because manufacturing costs.
-The necessary wiring for boost & wastegate control probably missing from the harness in the vanilla 1.6 variants.
-The rest of the Fiesta ST driveline has valuable components, such as Electronic Torque Vectoring/Limited Slip Diff.
-The ST has 4 disc brakes for when making big power. Because choose life!
Of course if it was even reasonably easy, you would see a lot of people doing it, however its a bit of a red herring.
I love tuning and modding cars as much as the next guy, but at the end of the day if I want Porsche performance it is cheaper in the long run to buy a Porsche.