This is not legal to do in Stock class autocrossing. There are noticeable horsepower gains that can be made by replacing the stock intake and exhaust components with higher flow parts. These items are available from many Miata vendors. With enough modifications, generally anything can be done. Monster Motorsports is known for their Ford 5.0 V8 conversion.
Rotaries present a problem, however.
The form factor just will not lend itself to dropping into a Miata engine bay. The gearbox linkages just won't fit without major modification to the interior as well as the engine bay. It may look like one, and it may be fast as all getout, but it won't handle like one. And the Miata is known for its handling. See the table of wheel weights.