Wow, that is...well..green..!
I would try something like mineral spirits on a small area perhaps inside the receiver. If it works I'm not sure how it will affect the parkerized finish underneath provided it's still there of course. If you can get the green off and find the original finish partly/mostly intact you could simply degrease and apply high temp engine enamel. I've done a couple Romy's that way which had bare spots in the finish and had good results. If the green doesn't budge I would consider making it camo if it were mine.
Of course this is the redneck way of going about things. I'm sure others here will have better suggestions for you.