The main purpose of that washer is to control the end play of the armature, if I understand correctly about what you're describing. There is a thrust load against it as the pinion gear engages the flywheel teeth, so choose something that won't wear or break during the operation of the starter. Phenolic plastic, Teflon, or nylon would be the only material I would consider if the original piece isn't available. How often are you willing to rebuild the starter if you choose wrong?
Jerry
