It's not cheap, but Turbo Tom is offering a 2.3L stroker kit for the 2.0 Zetec. It is on his
Focus Power website.
They have also made individual carb kits for the Zetec for a loong time. The Lotus/Caterham guys do it all the time. Check out
Burton Power and go to the Fuel System->Carburettors & Kits->Weber DCOE section.
BTW, the Duratec 4 cylinder is backwards from the Zetec 4 cylinder. (The intake is on the front by the radiator, and the exhaust is on the back by the firewall.)
HTH,
BP