Minirotty is the man to answer this question!

The exhaust is a good start. Most will have an aftermarket system by now, but ditching the air pulse system off the manifold is supposed to make a good improvement. The manifold will need rewelding and smoothing where it's been removed, but it's meant to be worthwhile.

People also say that porting and polishing the throttle housing helps a bit. Fit a panel filter and a chip and you should be going along quite nicely then.

After that, I reckon headwork and a cam change, but that's obviously a good bit more expensive.

I'm fairly new to them, so I'm sure someone may have other ideas, but it's a few basics to be going on with