Your 89 mustang has 88 horsepower, it went up to 105 in '91 to '93 mustangs. I've heard if you weld a bung into the older mustang 2.3L header (same engine) it will work fine. With a computer controlled 2.3L mustang it won't be cheap to get it up to V6 power levels naturally aspirated (at least compared to the new 3.8L's horsepower) and with the high revving you'd have to do with the engine and lack of cubic inches you won't have much torque. Your best bet for "beefing it up" would be to get a turbo and all related stuff from like a Thunderbird turbo coupe or even better an SVO mustang (made '84 to '86) and to get an intercooler designed for the '86 SVO. Stock '86 Mustang SVO's made 200 horsepower with low boost on that 2.3L. I think your mustang has a mass air metering system (i may be wrong), my friend's has mass air on his '89 so you could add a better camshaft, better exhaust, get your upper and lower intake manifold extrude honed (
www.extrudehone.com ), get a throttle body from a V6 thunderbird (i've heard it'll fit and give you more airflow), use an earlier mustang header or maybe one from a newer ranger 2.5L (they have headers stock) and a better air intake tube and filter and you should have around 140 horsepower which isn't all that bad, and it'll still be drivable on the street. That with better gears (like 4.10) and your mustang would have good acceleration, and with GT suspension would handle better than a GT because of less frontend weight (about 300 lbs. lighter directly over the front wheels i think.)
*cheap mod tip - in the middle of the intake tube going from the throttle body to the mass air flow sensor is a big round thing that has a Ford insignia inside an oval. this is the air intake muffler, take this off, take an exacto knife or something similar and cut out the air restriction on the inside (restriction in muffler cuts down diameter by like 1/2" or more) and then if you feel like it, cut off the bottom 1/2 of the actual air box to expose the air filter element to more air (as opposed to the 1 1/2" by 1 1/2" square tube that leads to another intake silencer leading to the fenderwell.
Good luck!
www.racerwalsh.com - order their catalog! they have TONS of stuff for 2.3L mustangs, in fact it'll probably blow your mind. Also,
www.powerheads.com has turbo 4 cylinder heads which i believe could also be used on a naturally aspirated 2.3L, and the head is CNC ported so it flows really well.
------------------
'84 Mustang 5.0 LX
My car