You might want to have a browse on www.ukfiestanetwork.com as some of the mk4 1.25 guys have done conversions.

From what I know, engine mounts are the same so you need engine, loom and ecu. Probably a bit more obviously but i'm not the best person to comment on that.

However, i'm having a modded 1.7 fitted in a couple of weeks to my ZS!