Realistically, its very hard to extract any real power from either the 1.6 or 1.8. The MX5 is quite capable of snorting alot more fuel than you'd expect, and financially at least, a
small turbo (you dont want to ruin the balance of the car), or if you can afford to run it, a supercharger is the only way forward. Look to spend upwards of £1000 for a turbo kit, and a fair whack more for a supercharger.
You might get 3-5bhp from a decent cold air kit, then mabey another 2 from a de-cat system, torque wont improve much though... And I think I've tried just about everything so far!