Ron,
That's called "patina" in collector's lingo, just a sign of the age of the item. Most like that appearance and leave it as is.
Some use an auto wax to shine it up, but those numbers are easily wiped right off the engine. Most collectors also don't like the polished appearance - again better left as is. You could give it a gentle wash with warm water and dish detergent, but again be careful around the number 152 and the NYC logo.
That engine is in really nice shape by the way. I would leave it as is.
Jim