A standard mold (mould) abatement solution is a 10% bleach in water. Add 1 part common laundry bleach (chlorox or any brand) to 9 parts water. Spray it on, let it sit for a while then rinse thoroughly. It really depends how bad the mold is. If you can see it all over the interior, I would toss the helmet, if it simply smells a bit, then try the bleach. You will have to dry it thoroughly after multiple rinses - consider putting the hat in front of a fan.
Most common molds feed on cellulose, and require moisture. Subtract the water and the mold will go into a dormant, spore state. I'm not aware of cellulose inside motorcycle helmets, but you might have a mildew bug feeding on the glue or something else in the lining.