As an NA car you are limited in what you can do and the 3S-GE is a pretty good engine to begin with. K&N filter in the standard air-box, trim away the plastic in the grille in front of the intake to give the air a big clear pathway in, exhaust headers, 2.5 pipe to a high-flow cat and a single straight thru muffler. You could advance the timing a little. This should get you down from mid-9's to high 8's in your 0-100 kph. If you are feeling brave and like to experiment try a JDM ECU from a gen-1 3S-GE and run on 98 RON fuel and that may give you another half a second, maybe.
Mitch.