1. K&N FIPK, I don't know if they make one for the 2.3, but I'm betting that you can fabricate one using a K&N cone-style filter without too much trouble. You can also go with a Cold-air intake system, I don't know of anybody that make them either, but they shouldn't be too difficult to fabricate.
2. Exhaust, There are literally thousands of choices here from nearly 100 brands, the most common include: Flowmaster, Dynomax, Edelbrock, Hooker, Thrush, MAC, Walker, Cherry-bomb(my favorite), and about a dozen styles(cat-back, header-back, off-road, glasspack, dual-chamber, dual outlet/inlet)
3. a T-5 tranny, if you have an automatic, consider switching to a T-5 manual, the performance and reliablitly increases are amazing.
4. Ignition system: Options here range from just upgrading the plugs and the coil, to a full-on MSD ignition box.
5. Camshafts, go down to your local parts house (Autozone, Advance Auto Parts, Pep Boys, etc.) and have them look up cam shafts for your car in the computer, see if they offer a high-performance option, you may be pleasantly suprised.
6. Intake setup, If street legality isn't important to you, you may want to look for the all-rare, but all-awesome Offenhauser 4-barrel intake for 2.3s, these are designed to allow you to bolt on a Holley or Edelbrock 4-barrel carb to your 2.3, Recomendations state not to go bigger than a 390cfm Holley, but a 400cfm Edelbrock is much cheaper and is of comparable quality.
7. If street legality is important, but you already have a carb 2.3 intake (2-barrel) you may want to consider looking around for an older Holley 2--barrel carb, or getting a Holley 2300 model (available in 350 and 500 cfm)
This is basically the same list I typed up in this thread: http://www.allfordmustangs.com/forum...&threadid=9706