Without forced induction you'd have to spend like 4k dollars to get 20hp.. or for 4k dollars get 120+ with forced induction. Check the NA Tech section, there's a guy named Andrew in australia who runs I think mid 14s stock. But I believe the australian model is the FS-ZE that is 170hp stock (so probably 130ish whp stock?)
Of course intake/exhaust is assumed, and on top of that things like an underdrive pulley (or the full lightweight pulley set from SR Motorsports that is crazy expensive), cams, port/polish intake manifold, standalone ECU (like a microtech which is like 1200 dollars)