The serial number should have been on the tub behind the passenger seat. There should be a couple holes about 3/4 the way up onto the wheel well where it faces the seat back.
You can try a serial number query over on the G503 site, or else on Wes Knettle's jeep site
http://www.willysmjeeps.com/v2/ (I think he collects data like that). The engine will not be the same one your Jeep came with, we got a rebuilt unit in a crate and swapped them out. It was possible that some of the earlier engines would have been converted to the later jeeps with the extra pulleys, alternator, and a few other minor changes. Most Jeep engines only lasted 25,000 to 35,000 miles before they were replaced. They were by no means a modern engine, tracing their mechanical and design heritage back to the 30s and early 40s.