If you want more power out of it, then your best bet it to fit ST200 stuff that comes up on eBay from time to time - Throttle body, cams, upper and lower inlet manifolds (UIM, LIM). Then fit an air filter (either a shrouded one, or one with a good cold air feed), remove the pre-cats, fit a decent exhaust and then get it mapped to suit the mods. You'll be getting towards 200bhp with all that fitted.

Unfortunately extracting power out of them isn't easy, or cheap, but it is possible.

It's also worth moving the IRMC (the bit that opens up the secondary inlet and gives you that 'kick') from the front of the engine to under the battery tray, otherwise they can get fried!