Pilots in my day, 50s to 90s, wore helmets because they were required
safety items. Had they not been required most jet fighter pilots would
have opted for a ball cap. Why? you might ask. The standard helmet
issued to Vietnam era fighter pilots weighed around 5 pounds. Pull max
G's and you got several things due to the helmet. One, your O2 mask
went down below your chin making it hard to talk to anybody. Two your
head was pulled into your lap and forget about twisting around to clear
your six. Why were these helmets so heavy? because we (USN) had a
helmet spec that required it to survive 40G's and resisit a 2 pound
spike dropped from ten feet above. No pilots had a voice in creating
these specs. Guess who did? Right, the helmet manufacturers.
Pre-Vietnam we wore Orange flight suits. Better to find your body
after a crash. One good thing the Vietnam war did for us was send all
these "Safety" pukes scurrying under their desks and for the most part
out of our hair. We wore really hot, sweaty, stinky Nomex flight
suits. Why? fire proof of course. I never heard of a cockpit fire in
a jet airplane but I'm sure there were some somewhere.

I don't own a P-51 but if I did I assure you I would not wear a nomex
flight suit or some damn over-speced helmet. If safety was really
paramount (the often chanted parable) I'd stay the hell out of a P-51
or anything which got me so far above ground thqt a fall might hurt. A
motorcycle? Forget that! More dangerous than smoking three packs a
day in a gasoline refinery. I'm for choice. You want a helmet, be my
guest. Just don't tell me I must also.