You can get induction kits, cam kits, exhaust systems etc, and no reason why you couldnt take it to an engine builder to have the head worked on. But TBH it really aint worth it, saw a 1.25 that had an exhaust system, ported head and induction kit and it only made 82bhp, its up to you though. Persoanly if you can afford the insurance premium a 1.7 (or maybe a 1.4?) transplant is a much more cost effective way of getting more power (I've heard of the conversion being done for £500, a pair of kent fast road cams alone cost approx £415, and the power gain wont be as large). Maybe not the answer you wanted but hope it helps.

Have a look here
http://www.pumaspeed.co.uk/car-Fiesta-Mk4-Zetec_1_1.jsp