to increase power and torque, you need to shove more air into the engine. but since you want to do it without forced induction, you have to allow the engine to breathe better.

easier and cheapest thing to do is to do the traditional bolt ons. granted they will only net you a solid 7-10 hp, the bolt ons are needed if you ever want to do any internal work.

basic bolt ons include: an aftermarket intake arm and filter, a more free flowing header, a bigger cat and b-pipe, and a free flowing muffler.

then if you want to do small internal work, you can get some more aggressive camshafts and a bigger intake manifold.