AFAIK, any UDP for the Zetec will work. eBay usually has a lot listed, but after a quick search didn't give to many listings. Most of the Zetec UDPs you'll find are 100mm, which works great. Any smaller UDP, you could run into problems (alternator not providing enough power, WP not circulating enough coolant, etc.)
If I had to choose the best bang for the buck mod on our Zetecs, its the UDP. You gain MPGs, faster reving through gears and you gain some HP(not very noticeable though.)
I have this one installed on mine: http://www.zxtuner.com/product.sc;js...&productId=129 But it says its back ordered.