Dont do a thing to it mate, if you look at the way some people on this forum attack new guns to make them look old, you could probably sell them a pre aged gas mask tin to go with their look. If you attack your tin with wire wool, prime it and spray it, it will look like a £40 brand new repro one (like mine). If the straps have had it, then replace them, other than that just wipe some gun oil on it to prevent more rust.
IMHO only
I'm just redoing mine right now actually as it was in a similar state to yours. I've stripped the paint off and am now removing the majority of the rust with my dremel. Then I intend to use some rust remover to clean it up before painting. I'm painting it with a couple of coats of spray on black primer and then painting it in RAL6006, the standard colour for most of the war.
Mines an early war one in apple green, I wouldn't dare do anything to mine, mainly as I likes it just the way it is 
that and I know at some point I'd like my moneys worth back for it, and spraying it in a modern paint (even if its not a huge amount of pennies worth) will instantly devalue it.
Mine was painted in some unknown paint over the rust so I have no such concern about ruining original paintwork. Just done the first coat of primer and its already looking better.
