OP here are your choices:

Stay NA & Keep motor un-touched:
Get better I/H/E - not sure what you have atm but you can invest $ 000s to getting a full toda or js racing setup
You will yeild next to nothing most likley

Stay NA & Open up motor
Install Cams and whatever else you can do - then tune the car after
of course you can do the fuel rail and all that but it wont add any numbers to your power (since thats what your asking)

Forced Induction
Pretty straight forward - but since you wanna stay NA this aint for you
Too bad though cos this would be your best option if you are chasing power