If you want to stick with N/A, then gasflowed head, decent ex.manifold + system, throttle bodies and a cam to suit should get you a good improvement. You could also go as far as to blueprint the bottom end and do things like lightened flywheel.

Most people just slap an RST lump in which has far more scope for big power. But trying to extract something out the N/A can be a bit more interesting....