This wouldn't just be a standalone business. It would be a weekend job to add to another wage. I used to work at North Weald along time ago before I was in The RAF. I remember seeing Mk4 and Mk10 series of helmets.
I've worked on both RAF and US helmets and the RAF offers more protection then the US ones.
The services I would look to offer would be Helmet painting and refurbishment.
I would also offer customisation and supply of Flight Suits and Flying Clothing.
But so far this is all just ideas looking at the possibility of doing it in the future.