Have you got money to burn? What is your budget?
Performance wise you could change the current manifold to something like the FSR Motorsports Street Fighter Turbo Manifold, change the turbo to a Garrett GTX3582R, assuming you are manual 6 speed change the ECU for something more modern like a MoTeC M130 or M150 (might be over kill) or Syvecs S6+ and as already mentioned a set of HKS 264 cams and Walbro 450LPH fuel pump.
As these cars are getting old what about maintenance? You could look at replacing all the suspension paraphernalia whilst it is still available, new water pump, new oil pump, new aftermarket diff, new OEM crank pulley and rebuilding your brakes. The list is endless