TBH, I don't know.

The first engines (6/98-10/99) had 2 idler pulleys on the cambelt at the 'front' of the engines (and a version of the alloy camcover). The engines after that had just the single idler at the top of the belt run. The idler bearing and centre mount was revised, as was its position on the head. The head casting was modified to add metal in the waterway for the bolt. The oilpump has always retained the bolt hole for the lower idler

I can't think of any engine mods related to the emission standards. The changes from ECC-1 to ECC-IV happened around 3/2001 (a few months before 'facelift'). This is when the post-cat lambda sensor was added and the PCM codes revised to reduce overall emissions during warm-up.

Oh, and another piece of (useless) information. The steel lower sump (oil pan) differs between the Focus and Mondeo engine. The gap for an exhaust pipe/cat is about 1cm narrower on the Mondeo, and it doesn't have the 'bump' at the pulley end. So a Focus sump wont fit into a Mondeo, but a Mondeo sump will fit in a Focus......although an ST170 or RS cat is a bit tight.