My understanding the primary difference is motorcycle helmets are not fire rated - meaning the liner. I think NJ state law is you cannot use a motorcycle helmet when racing in a car. I know I had to get the correct type last year to use at NJMSP.
I bought a g-force last year from
Twist Machine,
who by the way helped us a bunch with the Bob fundraiser last year. Good price, great service. I emailed him to see if it was in stock. I got return email later that night. I ordered it the next afternoon, then sent him an email afterwards about sizing, which he replied to quickly. Helmet came in 3 days later. Doesn't get better than that.