I made a fairly good replica of the Kato mask / costume once for Halloween, and it turned out pretty good. There's seems to be multiple versions of the mask used in the show, still photos, promotional shots, etc..
I went to a costume store, and bought one of the typical "masquerade" white face masks (like the Jabbawockeez wear) and I cut the outer shape and eye holes to match the sharp details shown above. I glued some thin flat styrene to the side temples to give it more volume like the image, and sculpted some Milliput around the eye holes to get that stylized shape.
Lined the inside with black felt, and painted the mask black. Worked out great.
The gas masks wouldn't be hard to scratch build from standard hobby / hardware store parts.