[84LX89GT]

You can also get walker exhaust systems for 2.3L's off of the turbo 4 cylinder SVO mustangs from '84 to '86. '86 SVO's used a single to dual exhaust system with a Y-pipe that splits into two outlets. I believe you can buy the Y-pipe that goes back to the cat-back exhaust area, and add the intermediate tubes and tailpipes with whatever mufflers you want. You'll get the cool chrome dual outlet tips as well. You can also go to some exhaust shops and get mandrel bent (flows 35% better than dimple-bent exhaust) exhaust made specifically for your car.

If you want to go fast, the easiest way is to find a '87-'88 thunderbird turbo coupe, '84-'86 Mustang SVO, '84 Mustang Turbo 4 cylinder, or any Merkur XR4Ti (German Ford that has turbo 2.3L) for a turbo/engine swap. these cars typically have good transmissions for the turbo engine (geared steeper for better takeoff) and have all the parts necessary. The '86 Mustang SVO 2.3L turbo put out 200 horsepower intercooled, which is a lot for a 4 cylinder in that era.
As mentioned Esslinger Engineering and Racer Walsh have everything engine related you could want, including 2.5L stroker kits, Camshafts, etc. etc.

Good luck on your new stang!