Well open up your hood and the big black ugly tube to the left that connects to your manifold (it should have the ford logo on it) is the silencer. However if you remove it dont hit any puddles and your car can overheat. You get a little more pick up by removing it.
As for modding four cyliner mustangs you can also add a turbo. Very cool especially if you want to kick v-8 *** with a four cylinder. Have you heard of a SVO mustang???? I dont know exactly what you need to convert to a turbo 4 ( besides the turbo) but Im sure you can talk to ford and get parts through them or find a TurboCoupe thunderbird and swap the engines. ANd if you find a TurboCoupe you can use practicaly the entire driveline.
Here are some websites
http://rothfam.com/svo/
http://www.corral.net/svooa/
This site should help alot
http://www.turboford.org/