Try looking at the shit that toyota put out for those. That might give you an idea on what you can do. I think that they supply sports exhausts. Not sure how good they are though.
dont just put a pod filter on, get a proper setup including cool air intake plumbing and a sealed / heat sheilded box for the pod to go into
exhaust has been covered to death but for the 1 billionth time around a bout a 2.25" system for a 1.8litre n/a engine. mild steel is fine, dont worry about stainless or mandrel bends pressbends will be fine, its not like u have a F1 car. get a proper system done that goes from the headers back, possibly even get extractors but why you'r only gonna see maybe 5kw peak power output difference with a pod filter and exhaust.