Search before posting, there’s many forums in the “420a bolt-on tech” that would be helpful to your question. The only headers available for our cars are going to be eBay ones, Spelab, SRS tuning, Flashark, etc. Megan racing used to make them but...
EGR would be a good thing to check, a stuck open EGR can cause the car to stutter or die at idle. Vacuum lines would be another thing to check. Depending on your state laws and plans for the car you could always do an EGR delete, cheap and simple...