the Zetec engines are very tunable. many companies do off the shelf parts for them. cam's throttle bodies manifolds (exhaust and inlet). the possibilities are endless. you can buy kits to turbo charge them too.
having the 2L rules out some options however, as with the 1.8L you can use the 2L TB and cams as i did in my old Mondeo. you would also need to remap it after doing this.
the zetec is often used in conversions and kit cars as it is a relatively easy engine to work on and is easy to obtain parts for.
the simplest way to gain power (and cheapest) is exhaust and induction kit. from there you need to think about a different type of inlet (different manifold, bigger throttle body, etc etc.)
remaps work well on these engines if you've got the right parts on them, and avoid "tuning chips" or piggyback chips. also i did see on ebay a while back a "tuning pack" and all that was was a resistor that you put onto the MAF sensor to up the fuelling. also a no no.
be careful when choosing parts as there are several different variants of the zetec engine and while most parts are compatible, some aren't. all the best!!!