--UPDATE (01/08/18) --
Added a new file with a narrower mouth area.
This helmet was modelled based off of concept art by Jason Pastrana: (www.jasonpastrana.com)
Because there were no front/side views, I struggled to get the correct shape - to solve this I used a character model generated by MakeHuman which really helped to make the helmet more ergonomic.
The chin piece is designed to be a separate part, so I would recommend attaching it to the helmet using something like elastic, so that it stays in the correct place, but also allows for some movement.