You can buy adjustable cam gears for the Mazda Miata, they are the same as ours. I think there are a few companies that make them.
Corksport sells WEB Cams, they have a variety of specs to choose from, or you can have custom sets ground. I think they run $450 or so.
All the intake manifolds are the same on the capri, turbo/nonturbo etc. Honestly if you are going for N/A I'd only portmatch the ports on the gasket and the head, and knifedge the dividers. That'll maximize runner velocity, giving you better torque which is what you want. Maybe look into a slightly larger throttlebody and a good solid intake system. Make sure you're tuned up well and run some Seafoam through the intake manifold to clean the runners and the tops of the valves.
I'm not sure on the N/A header, I know there was a company that made and sold them on eBay (they were painted/coated blue IIRC) but they have not appeared on there for some time, I dont think they are being made anymore. If you cant find anything, grab a Honda. I forget which engine it is, but its the very common one...lol sorry if its a bit vague I can get more details if you need. The ports match up almost identically, it just needs re-drilling of some of the bolt holes. Under engine clearance should not be an issue.
Hope this helps.
-Rocketman