Car: 2002 Lancer ES

I know this isn't the first time it's happened to someone, but here's the story:

CEL comes on and says fault code p0421. I take it to the dealer and, as I feared, they said the first cat is bad. Too bad mine has over 100K...

I've seen the headers w/o the first cat being sold for way under 300 dollars, like most of the cat/exhaust manifold assemblies that I've seen online.

The question is, can I run a header (without the cat) as long as it has place for the o2 sensor, and not foul up the computer/o2 sensors?

Like this one for instance. BTW, is it street legal?

Will a CEL Eliminator work with the header?

Thanks