Header ?
 

On my 97 DX Coupe, I want to replace the header with a more free flowing one, but the problem is that the catalyst is on the header, so if I buy an aftermarket one it won't pass smog because I live in south cali. Could I buy the aftermarket header, then get custom downpiping with a high-flow cat somewhere in there and still pass smog?

yup

What you can do is get the cat from a 96-00 EX. They are made with the flange on the cat itself so you can just connect the header to that. Maybe check the junkyards....or the forums...maybe ebay, thepartstrader.com etc etc.

put on the aftermarket header and then right before you get tested put on the OEM one and you should be all set or do they check them at random, but here they do a car inspection thing once a year