As already said, save and buy a GTi!
Enjoy the car by all means change the wheels/suspension/brakes. Your not gonna get much if anything out the engine. Exhaust will make it sound nice (+2bhp), induction kit might make it sound noise (0bhp gain).
Exhaust manifold on POTN - I can't find it
CAMS - Don't bother lol.
So in short if you want a quicker car buy one with a bigger engine. Don't waste your money on a 1.4! It's cheaper to buy a 2l 8v gti and insure it then buy a 1.4 and get it anywhere near as quick. TRUST US ALL WHO TELL YOU!