Hello. I have a similar helmet although not in as “brittle” condition. My example suffered several cracks around the face plate during shipping (to tightly packed) so I could see this was just the start of the deterioration. I applied a liberal coating of silicone over all the rubber surface. The idea being to apply natural latex over this and paint it with matt model paint similar to original green rubber finish. The latex did not take and had to be removed but the paint DID adhere to the silicone. The result looks fairly good compared to the alternative. I also pursued the rubber treatment question both with the Smithsonian Institute and our own BC Royal Museum. There is no method of preserving old rubber items other than storing them in a vacuum. I even contacted the makers of Armourall regarding the formulation of their product and its effect on old rubber items. Although they say it does not contain Formalin (formaldahyde) , it does contain water. I know some use and some don’t.