The stock NC chassis has a front brace bar and a thin sheet metal cross-brace underneath to stiffen the car and improve handling. If you find the stock bracing isn't sufficient there are a number of aftermarket options that could improve things. The stock bracing is quite thin and flexible and made to a budget, whilst some of these aftermarket options look almost industrial. Of course adding more substantial bracing is also adding weight, and bracing the differential will increase the noise and vibration transmitted through the car. Though this is unlikely an issue for a track based setup. Here are some of the chassis brace options available, including the factory Mazdaspeed option.
AutoEXE
Beatrush (Laile)
IL Motorsport
Okuyama Carbing
Mazdaspeed
Mazdaspeed
Mazdaspeed
Nielex
Nielex
Odula
TB Performance
Ultra Racing
Ultra Racing
HKS Kansai Service
"RPA" is one of a couple of options from China
Also from China, "KF3" or "KX-F3"
Again from China "TCR" brace set
Unknown from China