I wouldn,t mount anything on a helmet that could fracture the helmet in a crash.
Typical helmet mounts are either straps or 2-sided tape with a plastic subplate, the mount is not strong enough to affect the helmet body. Not easy (if even possible) to use one in an SCTA car.
A real problem with the GoPro is that the clevis on the camera cover housing is poorly made. No root radii, and cheap plastic, and bores are skewed. They will snap off over time, which has happened to many people so far. Should run a tether if mounted outside.
Cameras that require HDD's won't work for auto racing. I lost a lot footage using a cage mounted camera. The HDD shuts down to protect itself.