Welcome to the board!
You'll have to check the specific regulations of whatever group you want to run with. There are several different groups that run at MSR: Apex Driving Academy, NASA, The Driver's Edge, Porsche Club of America, BMWCCA, etc. Every one has different rules regarding open cars. Some require a rollbar, some require a rollbar and the driver's heolmeted head be a minimum of 2" below the top of the rollbar (hard to do in a stock Miata unless you're a midget), some don't require a bar at all.
I would suggest you start with the website of whatever group you want to run with and find out what their specific requirements are.