2001 Tubular header

The 99 and 00 Miatas have cast iron headers, the 01'+ headers are tubular. Better flowing and considerably lighter. For the non California motor, it's a drop in part. You will also need the EGR pipe as the fitting to the header changes from a male end to a female. These look gray because I painted them with some silver ultra hi temp ceramic spray paint to clean them up a little.
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The install was not all that difficult but conditions are tight under the car making for some creative leverage. I soaked all the bolts in PB Blaster so I did not have any real trouble removing anything. Gear ratchets help a lot too. The most difficulty I had was getting the three bolts at the bottom of the header off (and back on) There is not a lot of room under there to swing a tool. Wobble extensions helped. Also, when everything was unbolted I had to undo the clamp that held the front pipe to the bell housing in order to get enough clearance for the studs on the header up top.

Gaskets needed:
PN: BPM1-40-450A header to front pipe gasket
PN: BPD3-13-460 Head to header gasket
PN: JE06-20-306 EGR Pipe Gasket

Hi Temp Paint used on header Link

Note: My install was NOT for a California car. If you have what is pictured in the Cali Exhaust scan below, Old Putz from miata.net offers the following information:

If you have a Cal-spec '99. The midpipe already has an 02 sensor bung right up front at the bend, where it is on all the other NBs. From the factory, this is where the second 02 sensor goes. After ditching the boat anchor cast iron manifold (with or without the Cal-spec primary cat) this becomes the place where you put the first 02 sensor.
In short, regardless of whether you have a Fed-spec or Cal-spec '99 or '00, you do not need to replace the midpipe when you switch to an '01 or later stock manifold. All you will need are the tubular exhaust manifold, the heat shields, and a bung to weld downstream of the main catalytic converter. You can use the Racing Beat two cable extension set (part number 56003, includes the bung, and runs about $80) or make your own extensions.