I have an 02 Lancer Es, and as you know the oem headers come attached to the cat. I only have one cat, which is the front one, could I just buy new headers and a new cat to replace the oem ones or would there be some sort of issue.

 

Your wording is a little off...the 4g94 has 2 cats, one integrated in the exhaust manifold, and one regular cat. If you get aftermarket header, you will get a check engine light, so you'll need to trick the first sensor (spacer), or reprogram the ECU. Depending on where you live this may cause issues with emissions testing.