Well, I'm kind of crafty and most of the time I just go full steam ahead even if I don't have a plan - LOL - so I'd probably try a few things to see what I liked.
You could build up some areas of the masks by using Great Stuff foam in little dribbles, spots & textures. It's paintable with acrylic paint, as I've discovered in my crafting. You could add boils, make wounds or change cheekbone shape, nose shape, etc this way.
You could also get some modpodge & some cotton balls or paper towels or tp and start layering on a few layers to the mask that you could paint to look decrepit. I did the cotton balls on a skull method & there are a couple of pics in my album, plus there's tons of how-to's here & on you-tube for that.
You could poke holes in the hair part & insert fake hair, Spanish moss or strands of twine to change up the look - as long as it looks deadish or rotten, it'll fit right in. You could even glue a fake brain into a portion of the hair & make it look exposed, like that's how he got turned into a zombie, KWIM?