You have a few choices but all will cost a fair bit if you want a significant increase in power
1) conventional tuning, cams, headwork, inlet, exhaust and some way of setting the fuelling to suit.
2) turbo charge what you have
3) Scorpio Cosworth V6 conversions, V8 or Cossie. Duratec etc.
You could start by getting a power run done on a dyno and see what you have compared to what it is supposed to have.