Well if you're going to buy an ECU, why not buy the correct one? If for instance you fitted a 2L Zetec in a 1.8 Escort you use all the original components it's got therefore you don't need to worry about PATS (Fords immobiliser system). If you have to buy an ECU, buy a 2L and do the appropriate wire changes on the ECU plug but the ECU would have to be Pre Pats, something like pre '92 Mondeo.