Hi and welcome.
Availability. MK1's are rarer than MK2's. MK1 coupe's even rarer. No other reason. Except maybe the MK2's look better (personal opinion.
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The S2000 motor is a fantastic motor. But very expensive to do properly in a Escort. Very expensive. Hope you have taken this into consideration. If you have and still want to go ahead, the MK1 would be my choice because of the pre pollution laws of the 1975 and older cars. Which makes the conversion hassle free when it comes to engineering/registration.
This forum is not the best place for all things Escort to be honest. Search for UK based forums where you will be much better advised than here.
A recent UK based conversion below. Again, very expensive.
https://www.fastcar.co.uk/cars/f20c-...rd-escort-mk2/