I would like to start out by saying I know nothing about Kias.
That said, it sounds similar to the resonance chambers on Ford Probes and Mazda 626's, so that is what I'll refer to it as. The point of this is to somewhat change the pulses at certain engine speeds. on the f2's that I have experience with the resonance chamber adds 2-3 lbs of torue around 2-3k. Removing it causes you to lose that extra torque, but gain it back as horsepower in the top end.
So I would say that you could probably just do almost anything to your car, as long as it's airtight. This is most likely the reason that it ran so badly before. I'm guessing that the flow sensor (VAF, MAF, etc.) is on the air filter box, so the hole in the resonance chamber was allowing unmetered air in, which is why all those other wild things started happening.
As far as a replacement part, I keep thinking of a peanut butter jar, and use JB Weld or some sort of epoxy and use that as your new resonance chamber. Or you could use a piece of plastic from some container and glue it over the hole in your current resonance chamber.
Hopefully something in this helps you!
Edited by soobenthusiast, 17 March 2011 - 10:17 AM.