You can do lots with the Zetec motor. They respond very well to cams head porting elimination of the cat via race header or test pipe and other bolt-ons.

The biggest hurdle you will stumble on is fuel management. Ford uses a returnless fuel system which makes conventional methods a pain in the ass. Fortunately there are many ways to remedy this problem depending on what you want to do with the car and how much you are willing to spend. Generally the stock fuel system will be enough for most mild N/A street applications. When you start looking at more radical cam specs forced induction or nitrous you will really want to get more control of fuel management.

I suggest you go to www.focaljet.com for more info on what you can do to the motor. You have to dig through some shit but there are some very good reviews and dynos on the majority of the aftermarket stuff out there.