I haven't done any mods of that nature.. but since true function doesn't sound like its a major consideration...
I would say you have a number of choices on how to go.
the easiest would be to just put a "rebellion" style paint job on it with that in mind you can go simple or complex but I would do layered masking for the paint job for instance say you want some red rebellion symbols on it either paint it red, then make a insignia template and put your insignia "masks" in place and paint white over the top (or vice versa" have a punched out rebellion insignia template, and use it to mask what you DON'T want to paint.
if you want to put the "ridge" on the helmet, you could make it out of a number of materials, depending on what you have to work with. wood would be readily available but a bit problematic, styrene plastic or rigid foam might work fiberglass and resin might look the best but would be a problem to work with if you have never done it.
with a combination of say wood blocks and paper mache type material you could make something that looks ok but the surface likely would be seriously wrong, so after you get the proper shape, I would paint it with a couple coats of polyurethane varnish, then pain over the top of that to hide the poly. then possibly spray a thin coat of poly as a "sealing" coat as an option much like the "clearcoat" on peoples cars to give the paint "depth"