Still having trouble with that Cerbera Speed Twelve huh?
NA = Naturally Aspirated. The other tuning options are Turbo and Supercharger. If you look at the tuning shop at the dealer that you're taking your car too, you'll see a "turbo" (purple) button and an "NA" (orange) button that you can click on to purchase upgrades. These three types of upgrades - NA, Turbo, and Supercharger, all add HP to your car in different ways, but only one of the three may be equipped at any particular time.
In order for a car to participate in the NA races, it cannot have any Turbo upgrades. I belive it also needs to be ABLE to be tuned to NA if you leave it stock. An NSX is a good choice for the race, I used a Dodge Ram with a State 2 and some other upgrades (but again, turbo and supercharger aren't eligible).