Honestly, there isn't much more that you're going to get out of that engine without investing a disproportionate sum into it. Headers and an exhaust are about all I would bother doing with it.
I would not do a K&N-type open, oiled air filter setup, even though you will see a HP increase with it. In my experience, even with proper maintenance, they let debris through that the OEM paper filter would have stopped. I dumped my K&N FIPK intake kit and went back to the OEM intake box for this reason.
Where you will really get your "power" is in making sure that your axle gearing is optimal for your tire size and intended usage. A 2.4 is never going to be a stoplight screamer, but if you have it geared properly you will be fine on the interstate and on the trails.
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