KO4 VAG...Audi, Mk4 golf etc... is good for 260 to 280 bhp flat out on a 1.8litre so should get you to 250 on a ZETEC I would have thought, after that you can upgrade to GT28rs or borg warner for 400 bhp.

You can buy whole engines for £300 so a KO4 should be peanuts.