The temp sender MUST be the one that was used originally, as it is likely to be a different resistance and so give false reading, so use the xr3i one, even if you need to make a adaptor to join the original temp sender to the new engine stat housing, unless by sheer good luck the new engine's one is the same resistance ?
I'm assuming you're using the std coilpack on the zetec ?, in which case see if there was a mk4 with a coilpack and use the tacho out of that is easiest way, my memory seems to tell me there was such a car/engine ?
tabetha