as said, zetec blacktop will be cheaper to buy, not massive hassle to fit but still messing about.... ( youl need from a 1.6/1.8 escort/fiesta zetec E silvertop: sump, oil pick-up, flywheel/clutch) if using a blacktop then engine mount maybe easier to mod using a mix of fiesta/2.0 focus parts. the exhaust will need modding as its on front. and of course ecu, which i doubt will plug straight in. you would have to check the exact pin location and count first as theres at least 3 variations on mk4/5 fiestas alone.

however, if you want a nippy fiesta that can be easily turned round in a weekend then stick the puma lump straight in. a bit dearer initially like you say due to demand, most places round here want around £600 for the engine alone!!! so £750 supply & fit isnt too sad. but its the ease of the job thats the benefit.

overall, its tricky, both similar power, id say the 2.0 is stronger and cheaper more versatile to modify/tune at a later date ie turbo or throttle bodies or carbs etc. the 1.7 although better than its smaller brothers isnt so forgiving to abuse and requires loads of cash for high power. but it is such a quick, easy and relatively cheap swap if you want a nippy reliable fez.

How about a fiesta ST engine? I think a chap called olly from OC motorsport did this not long ago, nice tidy swap too! just an idea.....