Im with Shane on this, get a proper gearbox.
But in all honesty, unless your prepared to spend a fair bit of money.. there isnt much, get a snail.
I know its a cop out. but its the best way
But the basics are as nath explained, get the engine to breath better, intake exhaust, advance the timing. This can be done on a reasonable budget. But gains are minimal.
Any more from there, your going to throw a couple of grand money at cams, cam gears, ECU and tune, for considerable, but not huge gains.
Futher from there, your looking at ripping the engine out.
If you wanna go fast in a straight line, buy a rotor, they are still P Plate legal