I am amazed that your engine is nickel plated! Those eBay pics sure had me convinced it was a brass version!
Glad your engine survived sub-minimal packaging as well!
I was like you and thought this to be a brass engine for sure, but then I found a streak of bright silver and thought it looked to be nickeled.
Now I'm not so sure and don't want to go scratching around to find out without taking it apart and getting to a proper cleaning. The streak between rivets that looked to be nickel, may just be some solder overrun as pushing the thumbnail test a little further out away from the rivet, the metal again looks more like old brass.
See the original silvery streak here:
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Not sure what to clean the boiler with, in the way of solvents, but pretty sure I don't want to actually polish it as I do prefer the patina of dull brass to the glare of bright brass!
Hummmmmm!?!?