air intake: will help the engine suck air in easier and will provide a louder intake roar, will add a few horses higher up in the rev range
headers: will help expell hot exhaust gasses from the engine quicker and will probably be more uniform in design which will also help your engine breath better, this too will add a few horses higher up in the rev range
irriduim plugs: the concept is perfect on these plugs but in all honesty they do nothing on such cars, they require spark amplification before these type of plugs show their true colours. you can get these if you want a long lasting plug otherwise they are no better for this application than plain plugs.
