What your talking about is a ZVH conversion, which for a Mk5 xr3i would be a waste of time as the zetec 16v has the better head than the 8v CVH. And yes there is a lot of work involved if you do try it.

It would be infinately easier to find a 2.0 Zetec from a Mondie and drop that in cus you should be able to put it in without any major hassle and only really need to map the ECU and change a few of the ancillaries. Plus you can turbo it for about 2k-3k and bobs your mothers uncle you have a cossie replica that goes like one.