i don't see how it would be much different from prepping a jango helmet. though i'd be worried at how well the helmet has been holding up after all these years, sometimes the plastic can take on quite the cantankerous personality. but so long as it hasn't "dried out" and cracked you should be able to heat it up and gently work out any warps that may be present. once that's done it's prep time for painting. i'd suggest using 220 grit to take off the original paint. then 400, and 800 grit to get the surface super smooth to prime. once you primer it, give it a good look over, if there's any spots that are still rough, go back to 400 grit and then 800 again then re-prime. sand gently over the entire helmet again with 800, one more light coat of primer then go over it all one more time gently with 1000 or even 1200 grit. at this point your surface should be ready for paint, a couple light coats of whatever hi-gloss red you're using, then let it fully dry. search for imperfections and lightly sand them out with 1200. another couple coats of red, and again let it fully dry for a couple days. dust it off with a microfiber cloth when you're ready to clear coat. a good hi-gloss clear coat is needed here, i'd suggest 2-4 coats depending on what kind you get and the illusion depth it creates.
your arm may be very sore from all the sanding, but the finished product will be well worth it.
