Wow...Sucksville, Population: you.
I know that cat piss is quite aromatic...having an overpopulation of strays in my area.....and getting rid of it can be troublesome. After you have cleaned out any solid and liquid waist, and as
Thane said, sterilized the inside w/ some rubbing alcohol, you could liberally spray some Febreze or Nutra-air to eliminate the smell and then just let the helmet air out.......just make sure the cat doesn't get to it again.