How much power gained if i were supposed to rebuild my current stock B16a2 motor with just aftermarket like JE pistons and Manley rods? Supposed those were the only parts that i replaced during the rebuild, how much power increased would i gained from those 2 parts alone??

 

Rods don't increase or impact power, just what power is safe to push to.

Pistons won't change much by themselves, and it's also important to consider what the compression of those pistons is which you don't have listed right now.

 

Then how do one increased power by working on the internal part of the motor without going boost?

 

Increase displacement, increase compression, increase camshaft lift and duration, larger throttle body, quality header, larger exhaust pipe diameter, increase intake manifold plenum volume, and CNC cylinder head work will all help make more power naturally aspirated.