I'm doing the same to a TCS. From my understanding, the 'y' in the S2 frame is narrower than a TCS frame, so I don't know what's involved there in fitting a Zetec. On the TCS, the frame needs to be "notched" for the alt. belt. The TCS bell housing bolts directly to the Zetec, so, you'll need that, or a adapter. Throw in a cable shifter, different routing for coolant piping, engine mounts, etc.
There have been many more Toyota engine swaps into the S2. One Yahoo lister was making parts for the swap for a while, don't know if he still is. I seem to remember seeing a Toyota adapter plate for sale in the last few days - maybe on this site. There is/was a NG3 and shifter setup available here also.
That is just some general info, I'm sure others will chime in with more detailed information.
Good luck with your endeavor!!