You can get a racing puma intake manifold + induction kit and a decent exhaust manifold and stainless exhaust system to get it up to about 130Bhp or there abouts which is the same as the standard (blacktop) zetec. Ill bet piper will do you some cams too which could see upwards of 150Bhp (thats same as fiesta ST). Get the cat removed or replaced with a freer flowing 'sports cat' and you could see almost 170bhp but I wouldnt want to take it any further than that. Alternatively you could just chuck in a 2.0 out of anothr focus / mondeo and give it some forced induction. Theres over 200bhp without even thinking