Using a dremmel tool maybe the best way. Certain tools allow you to cut and sand into the helmet and then I have heard of using a soldering iron on the marking you have made to make a melted/scratched effect.
i use this technique on my scale cars, maybe you can try it too, what i do is spray my base coat(silver metallic) let dry completely then i use rave hairspray(not the one in aerosol, the one in the pump meister) then let dry, and add the top coat and let dry, then with a tooth pick i begin to scratch as desired, when you finish you can seal with matte clear coat or pledge with future, i hope this helps
Are you wanting to deep scratch into the bio or just in the paint work???
Deep scratches as said dremmel is your friend, great tool. For scratches in paint work, again as said a base coat of silver, then you can actually my masking fluid, sort of like a latex. Paint the desired scratches and then top coat paint. once dry you can then pick of the masking fluid leaving proper scratches in the paint.