The hinge can be made quite simply, a coupe of ordinary door hinges will do not to large, measure the top of the faceplate where it meets the helmet, and size you hinge to fit in the middle. the problem that faces most is how to move the faceplate up and back at the same time . You can achieve the by mounting one hinge plate to the other using fast set epoxy don't get any on the hinge itself, you want both to move freely - so now you've got two hinges stuck together - mount one free end to the face plate and the other to the helmet and if you've not managed to stick them together the wrong way round !! trust me it can be done !! and you do feel a fool ... anyway ... you will now have a faceplate that moves up and back . The tricky bit is now to motorise this, I'm not a great one with servos , there are those here better than me . so have a search for motorised faceplates , you mount the servo in the helmet and mount a rod from the actuator arm to the servo via flexible joints or couplings , ( think R/C planes here ) add power to the servo , the servo will make the arm turn, pushing the rod. which makes the faceplate swing back against the hinge and viola !! it open the helmet !! ,there is slightly more to it than that but in simple terms that what you would do .
