If its in the plastic, its harder to remove. If I were you, I would try an odor absorber. After hurricane Katrina, everyone's refrigerators were foul smelling because food had been rotting inside for weeks or even months. The smells were thought impossible to remove. I had already replaced my old refrigerator, but I was determined to remove the smells so I could use it as a back up fridge in my garage. I bought these canisters of odor absorbers from Home Depot in the cleaner aisles. It took me a a couple of weeks of leaving an absorber in the unit and replacing it each week, but it really did the trick. I do not know if it will work with your ABS helmet, but I assure you, it took a most horrible smell imaginable and sucked it out of that full sized refrigerator.
It was called OdoBan and they are about $3 a canister. Just something that might help you. Good luck trooper!