why do people always say save your money, buy a faster car etc theres nothing wrong with modding a 1.2
dont mod your 1.2 buy a faster car?! well why mod your 172 or 182 when you could just buy a faster car?
best way to make your 1.2 faster mate is to start with suspension and brakes, then it will be twice as fast in corners, then go for the usual air filter and cat back exhaust and a decat if you can be bothered with changing it at mot time.
next you want to save weight, strip out the back seats if you dont use them or if you want to be really hardcore strip the rear seats, rear doorcards, rear carpet, spare wheel and fit buckets!
another way to get these engines faster is plenty miles, pile the miles on it and give it a good thrashing when its up to temperature some times, will loosen it up!
then if your stull not happy you can go for what edde says, remap, cams, headwork etc.
EDIT: also dont know how much greif it is to convert it to a 16v engine