With a crossflow engine, I don't think headers really apply... Correct me if I'm wrong. But a good option to modifing your 2.0L is intake, exhaust, cam, cpu, and a good port polish, a polished intake manifold, polished throttle body, polished exhaust manifold, 3 degree vavle job. The vavle job and all that polishing should be done together at a good place where they've got a flow meter to test their polishing.
I've researched it quite a bit, as I'm in the same basket as you, stuck with a 2.0L 8v. Check back a bit in the posts, there's a thread where I asked the same question.