The outer side of engines generally doesn't get corroded... It's the inner parts that matter. You are at least going to have to pull the backplate to see what's going on inside. If it looks fine except for some gummed up castor oil, loosen it up with some fuel and give some rich runs. On the other hand, if you see ANY corrrosion, you're going to have to dismantle it, take away the rust with some very fine sandpaper and replace the bearings. Make sure you put the parts back in the same way they came out, they are mated to each other.