2.25" or 2.5 diameter exhaust
custom header(if you can make it)
cold air intake to under the bumper(check out 900aero.com,under DIY)
possible nitrous injection
slightly uprated injectors
I go with removing as much weight as possible. But that goes for any serious performance car.
That goes for engine rotating assembly too. Custom con rods, custom pistons, balanced crank. Maybe slight increase in displacement too. Then I'd cam the thing up, make it run to 8 or 9000. You'd need appropriate gearing to match, so that you could keep it in the powerband. Then I'd go with the headers, of course. Also on the top end, I'd open the thing up as much as possible, big valves, lots of porting. Then I'd do a custom intake manifold with individual throttle bodies.
It's not unfeasible to have an NA 2.0L make, say, 240hp, but it would take lots and lots of work. The Rover K-series and the new Cossie 2.3 in the latest Caterhams comes to mind - but Caterhams haul around less than half the weight of a c900....so it comes back to power to weight again.
There might be issues with revving a bigger displacement B202 or B212 but I don't know much more about piston to rod length ratios and all that. But generally, revolutions and displacement get you power in NA applications - look at BMW's V10 in the M5, any of Ferrari's V8s or the upcoming Audi RS4 V8. All in the 4.0L plus range, all running to at least 8000rpm. All expensive too!
I have an engine simular/calculator thats a few years old on my computer and it says the max speed of the b202 is 7000 RPM same with b212 cause they have the same stroke...btw Im soon going to get a 92 900 to mod...so bigger engine and rear handbrake...
also if needed I might install a nitrous system...for killing whatever if needed...