You can do most things to a NA engine, depends how far you wanna go and what cash you got.

First things to get you some power would be:

Air Filter - K&N, Pipercross etc.
Cam - Fast Road or Rally - Piper or Kent
Exhaust - 4 branch exhaust manifold and full system
Chip - Up the rev range
Rolling Road Set up - Costs about £100 but will finely tune the fuelling and give you a good readout.
Higher Octane fuel - try using optimax etx better octane rating.

Then depandant on what you wanna do:

Engine swap - 2.0 litre ZVH modeo engine or RS2000 engine all produce 150bhp standard (if your gonna do that just turbo the fucker)

NOS - as you said, I managed 75 bhp with no problem.

Depands what u wanna do and how much you got. The world is your oyster.