In some states/locales they inspect to see that cats are on the car. Doesn't matter what kind as long as they're there. Other places (like California) they must be OEM cats or be approved replacements. Approved replacements have a CARB EO #. Generally (but not always), the CARB EO # references a specific application, so a cat that is approved for Mustangs may not necessarily be usable on Marauders, even though the cars have the same engine.
I believe all the exhaust components have similar rules, but for sure these rules apply to headers. Probably mufflers also. I don't know about tailpipes, mid-pipes, x-pipes, h-pipes and such. If so, it would be necessary to get an approved complete system instead of piecing it together like some of us in non-emission-tested areas have done.
As for CARB approved exhaust upgrades for the MM, I don't think there are any yet. So you can gamble on a system getting approved between now and the time you have to be inspected or keep the stock system and swap back for the inspection. Lots of 'stang guys do that. There are other ways around the rules but we shouldn't discuss that in a public forum.