I'd say, if you're going for turbo power, use the 2l zetec, as it can take it easier, and is a heavier motor. If you want to stay n/a, use a 1.7l from a ford puma, can easily get them to about 150bhp. If you want to know about the 1.7 conversions, get onto www.zsoc.com as there are a lot of people on there who have put them into fiestas, so can advise you, and also point you in the right direction if you want someone to do the conversion for you

BTW, when I said 'zetec' I meant the 2litre from a mondy/focus.