The response is always the same sadly when somebody asks this question. "save for a better car" etc. This irritates me. Being a 1.2 16v owner, and knowing full well it's going to be between 1 and 2 years before I can afford my next motor, I have wondered how to get more power. It isn't pointless, and spending money isn't a waste. (To a certain degree.) This is your hobby, your car, your pride and joy, fair enough if you want a good price when you sell it don't modify it, if you're willing to accept that, no worries. Having said that it's not hard to put it back to standard.
Back on topic, If you're looking for a little extra improvement give your car a good servce, plugs, oil drain/flush/change, new leads & coilpack / clean the ones you've got with WD40, get new oil and air filters, don't think you can change the fuel filter.
Once you've done that look either a performance panel filter or if you have the money an inclosed induction kit with an air feed. After that there's not much else but the servicing will make a difference. One last thing you could try is weight reduction, stripping out unnecessary interior parts/loosing any subwoofers/crap in your boot.