I've heard that a modern Zetec engine fits quite nicely in a mk2. I also have a mk2, but as I'm only 18, a Zetec engine would put a huge dent in my wallet from the insurance.
To be honest though, I think the mk2 1.1 has enough as a top speed. I've maxed out the speedo in mine (110mph). So it will do at least that given a long enough bit of road. It's acceleration can be a bit disappointing at times though, so I guess more power would help that.