Hmm idk if there is a way to enable the helmet... I remember having it in some cases like in replays with cockpit cam, so it might be an option somewhere. Or it is hardcoded in which case the only other solution would be to use F5 cam (you can move around with WSADFGZCBV and shift/ctrl for different speeds), but that one cannot be saved per car.
ps: ah that F5 cam needs to be enabled first in "camera_onboard_free.ini" file - it's located in AC installation folder -> system -> cfg. You have to switch "SPHERICAL_COORDS= from 1 to 0 there to enable the F5 cam to move around.