How do you intend to paint the engine, with an air brush or by hand? If you are going to spray then try Tru Color as they have paints specific to rail roads.
No problem, glad to be of some help.
It might be easier to spread a small amount of aluminum foil inside the shell, attached with a little blob of silicone glue and pushed flat with a finger.
If painting the "inside of the shell" is just to create a light barrier, wouldn't having a good coat of primer, along with the final colors, block light from getting through?