if it's a cams-approved motorkhana, you need a helmet that complies with the same standards as a motorbike helmet
i just got my helmet out, the number is AS1698
mine's a full-faced $700 shoei, but i'm lucky. some of my mates race with open-faced $120 helmets, and really, for the occasional racer, that's more than enough
open faced helmets are crap ideas on bikes, because if you come off on your chin, you're likely to lose your jaw. as in the bottom half of it. such a danger isn't really present in a car, so it's fine