The head is actually very good on the ST170 so headwork is not absolutely necessary to crack 200bhp N/A, you just need to get it breathing a bit better. That means a better exhaust system from manifold all the way back...It's not as efficient as most people will tell you. It's tuned for mid range torque so swapping it over for a big bore system will give you top end gains and mid range losses. Add some wild cams (delete the VVT) and throttle bodies and you should be cracking 200bhp. High comp pistons will really help and you can go for a larger bore (up to 2070cc) but that's an expensive step and again, not 100% necessary.

Will Pedley is the man to speak to if you're serious about a N/A ST170.