Weight is a big advantage for the Duratec. If it's 30kg lighter, that's a big percentage in engine terms and a good weight saving in a car as light as a Seven. The question you have to ask yourself is, in a car as basic as yours, where else are you going to remove 30kg from one component? As Colin Chapman said, adding power makes you faster on the straights. Subtracting weight makes you faster everywhere.

Tuning options are big for the Duratec too. There is piles of stuff out there for it as it's used so widely. Mountune did a kit for the Fiesta ST (which used a 2.0 Duratec) that took it from 150PS to 165PS. Think that was just an exhaust, intake and remap. Keep in mind that most ST170 are usually in the low 160s in terms of horsepower and it suddenly becomes very close. Don't get me wrong, the Zetec is a good engine, especially in ST170 form, but it's a bit old-hat nowadays.

It does partly depend on gearbox compatibility though, as I am not sure on how the bolt patterns compare. That being said, loads of people run RWD Duratecs, so it must be possible!