There isnt alot you can do to a 4 cylinder exhaust to get a real deep sound like you can with a V-8, however I have an 89 coupe four cylinder with a Dynomax Super Flow muffler and 5.0 style tip that can get fairly deep around 2000 RPMs, sounds cool in parking garages
. As far as wheels go you can put anything on it that will fit (I have a set of ARE 5 spokes with 215/60/15 Firestone Firehawk GTA tires) Let me tell you, getting new rims on your stang makes a HUGE improvement on the looks! Other wise if you want the same V-8 sound you might as well install a V-8 engine in your car. Ya cant beat V-8 sound!!!
Headers can also be installed in your stang, I believe a header off a 4 cylinder ranger can be used and I have heard of long tube header installed on a 4 popper. Anyway I hope I helped and good luck on your car!