Your NB1 Miata uses 2 catalytic converters in states that complied with California's emissions standards.  Pennsylvania is one of them, along with NY, NJ, CT.  One catalytic converter (cat) was mounted in the exhaust manifold and the second was mounted under the car, about under your seat.  The first catalytic converted was used to clean up cold start emissions.  The computer running the engine used (2) O2 sensors to monitor performance of the catalytic converter in the exhaust manifold.  One before the cat and one after the cat.  When the delta in hydrocarbon readings got to close to each other, this indicated the cat was loosing its effectiveness.

So if I understand your question correctly, you want to install a manifold onto your NB1 that does not have a catalytic converter in it.  That is easy.  Buy a Racing Beat Header for your car.  The RB header does not have a cat.  Since the O2 sensors are in the engine bay to monitor the cat in the manifold, the wire to them are short.  The RB header comes with 2 wire extension harnesses that allow the O2 sensors to move underneath the car to monitor the second cat.  The factory computer will not know it's monitoring the lower, second cat.  The only additional mod you need to do is weld a fitting for the second O2 sensor behind the cat under the car.  Any one who can weld can do this for you.