The SVT conversion is the single best "bolt on" performance mod that can be made on the Zetec. The HP gain per $ spent will be less on any other bolt on. the SVT header has shown solid gains in the 11HP mark at the wheels. With a Tune, and Dyno tuning Adjustable Cam Gears are some of the best mods that can be done on the Zetec. The only other way to get a higher HP increase per $ spent will be going forced induction.
You can also waken up your car by new springs and suspension changes that are relatively cheap.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)