I had a K&N 57i on my old Mk1 1.2 (which, because its also an "Energy" unit, uses the same airbox/throttle body as the 1.4). It was, quite frankly, crap. I thought it was good at the time, but mainly because of the noise. Between that and the big wheels, the car was definitely slower. I dont think the case will be any different for you as it was for me back in 1998!
There are very few mods to increase power (and even then, you may never notice it): one of the best being a de-cat pipe. Ive been through mods like cams, porting, Facet valves, Vernier pulleys, exhausts, induction etc on all my Clios apart from the Williams. None of them really give proper VFM like buying a fast car in the first place!
Im not suggesting that you dont mod the car, but dont do it expecting a power increase. Major mods like a new cam, headwork and so on are possible and have been done on the 1.4, but youve got to ask why when you can buy a rip-roaring, fat arched, sorted suspensioned, bonnet bulged 16V for about £1500.