In short, you cant. To get to a real 125mph (not just on your clock) you need a dramatic increase in power that you wont get from the 1.2 16v. More importantly - even if you get the magic 125mph capability, how often will you use it??
I hardly ever drive fast, but I was going for a top speed run just this morning out of curiousity. On a totally deserted motorway, I was finding it really difficult to get over 125mph (it will go faster, but takes ages - never had a top speed yet). And thats in a tuned car with higher gearing and nearly 100bhp more than the 1.2 16v!! You see, areodynamics, wheels and gearing are just as important as power.
Its very difficult to extract significant power gains from NA engines like ours. Id be happy with the usual induction and exhaust mods to make it sound better and make it more "driveable". If youre looking beyond that, youve got to ask why not get a car that comes with - and is designed to take - the extra power youre after.