Being 100% up front with you it's just not worth the hassle. You'll need a new engine gearbox and breaks, you'll need to change most of the suspension to deal with the extra weigh when sitting an bigger roll bars for cornering.
The 1.8 back brakes are as big as the front brakes on your 1.2. Your standard 1.2 brakes would not be up to the job, non vented discs.
Without doing the brakes and suspension, it might go faster but will introduce its self to a tree at the first sign of a corner.
The electronics on modern cars means it's not a simple case of swapping things about, you will need to change all the engine wiring looms as the 1.8 uses a different loom and then try and get all the various modules in the car talking which if the engine comes from a car much newer or much older than your current car then its not going to play nicely.
Finally the insurance on your modified 1.2 will be massive compared to if you just insured a standard 1.8hgt mainly because the insurance company will be taking a gamble in giving you any cover at all, that the car is safe, the brakes and suspension are up to the task and that the wheels won't literally fall off.
So the best and cheapest way to do what you're proposing is to take your 1.2 and trade it in for a 1.8HGT but get some insurance quotes first as you'll need to know this before you dive in and get the 1.8.
It's also worth remembering the 1.8 punto is only 130hp and not that light for a car of its size, you can get a lot of more powerful faster family saloon cars which won't have the high insurance costs that go with having a Hot hatch.
When I was 19 I had a 1.8 Mondeo 16v zetec my friend at the time had he same engine in an escort xr3 the Mondeo was a little heavier but with some tweaks the engine output was the same and the weigh difference was not severe. He was paying 3 times the insurance I was paying, I'd upgraded from a 1.1 fiesta and the Mondeo was actually cheaper than that to insure.
Gone are the days of strapping on twin weber carbs 