There are a number of 'chrome finish' spray paints available at your local auto parts store. I would buy a few different cans and try them out on another surface. You might be surprised by how reflective they can be. And i'm sure with some light wet sanding, you should be able to punch up the mirror finish even further.
Prepping will be your biggest ally, as the chrome paint will amplify every ripple, bump or imperfection on the helmet's surface.