i own a 90 celica gt, and you and me my friend, share engines....and i hate the break it you, but there is virtually no aftermarket for the 5th/6th gen celicas...
you can chop off i think 3 or 4 inches from a honda civic short ram intake
you can custom make a cold air intake
exhaust has to be custom done by a shop...unless you've got a 3sgte 5th gen alltrac
i don't know where to get them, but i'm pretty sure there are headers out there....there is a turbo kit(around $2500) and a supercharger kit(around $3,000)
basically, swap in a 3sgte and you've got a much better aftermarket, with parts from HKS, JUN Auto Mechanic, several others
and 3rdWrldRcr, LCEngineering didn't appear to have anything for the 5SFE, they appear to just cater to the toyota trucks...
engine codes for 5th/6th gen celica's are as follows
ST - 4AFE (4AGE, 4AGZE are direct swaps, with the exception of wiring)
GT - 5SFE (3S-GE, 3S-GTE are direct, exception being the wiring)cannot be made all-wheel drive(at least cheaply)
GT-S(no GT-S for 6th gen) - 5SFE (3S-GE, 3S-GTE are direct, exception being the wiring) cannot be made all-wheel drive(at least cheaply)
All-Trac - 3SGTE, and all-wheel drive...
so there's your engine codes, and the answer on aftermarket support
however, suspension mods should work, i know struts/shocks are 5th/6th gen switchable, but strut bars, sway bars etc, probably aren't