The serial number is on a data plate on the right side of the firewall just below the lip of the cowl. It's held on by four screws, so it may be missing. If it is gone, there is no way to find the serial number unless you have some detailed history of the vehicle. on my 1946 willys there is another I.D. tag with matching number on the outside of the left front frame, just behind the fronr bumper
The specifications of a 1946 Willy's CJ1A, which is a jeep, are numerous. The jeep, for example, has four cylinders, a wheelbase of 80 inches and a Bore that is 3 and 1/8 inches long.
They both use the T-90 transmission and should bolt right up. There are differences in the bellhousing sizes and even the flywheel ring gear, but the available ring gears should fit all of the flywheels (not all of the bellhousings). There are three different ring gears that fit the flywheel (that I'm aware of) 97 tooth, 124 tooth, and a 129 tooth. The 129 tooth will not fit in the small bellhousing of some of the CJ2A's. Check the outside diameter of the ring gear and the inside diameter of the bellhousing before. -RB