I would advise finding a helmet that looks good with C&V bikes, but is built to modern standards. I started a thread on this waaaaay back but I don't recall what the results were, and helmets have changed anyway.

Does anyone have any suggestions? I will say that most of the helmets that are made to look like they're just hats look totally ridiculous. They have to scale the hat up too much and you end up looking like a cartoon character. I saw two guys out once in helmets with covers that looked like driving or pub caps, and it was totally comical.

edit: LOL! Here they are! They actually looked even worse in person, if that's possible! Another version.