Bottom line: You don't.
The Miata has 167 engine horsepower, to start with.
At the wheels, that translates to only around 130 or so.
So, by wanting to increase your horsepower by 50, you are actually wanting to increase it's output by almost 40%.
That doesn't come cheap.
Intake, headers, exhaust, will cost you around $1500. And only net you a gain of around 5-6 wheel horsepower.
And immediately void your warranties.
You could get a turbo kit, but for it to be properly set up, tuned, etc, will cost you far beyond $4000.
And still void your warranties. And MIGHT not even get you the 50 hp you're shooting for.