Hey nice bike mate, well done! If that green is the original color (and I'm guessing that it is) then leave it alone and just tidy it up, those old Rickman's are very collectable now.
Mick is right about the frame, I was always told that you shouldn't chrome frames as it makes them brittle, but I don't know the metalurgical reason why. Get it re-nickled, and keep it original, and nickle plating is cheaper than chrome anyway.
Back in the early 1980's, I could have bought an orange Rickman CB750 from Vic Wreckers for 900 bucks, it was leaning up against the wall looking very forlorn, and I thought about it, but didn't act. I wish I had. Cheers, Terry.