I have to ask what wheels are on your engine? Are they the 8 spoke wheels, and the bottom plate says Lionel Jr.? If that is what you have, they don't have spacers, and the drive rods slip onto the hub, which was peened over at the factory. I have three 1681's I am restoring at present time. A photo of the drive wheel with hub would help. Which would give a possible fix.
The possible fix if the hub has been torn up, is to drill and tap down the center of the hub with a 3/48th tap, which would accept a hex head screw part number 700E-45c which is a short threaded 3/48th" screw for attaching side rods. That is your only alternative, depending on the condition of the hub, if the side rods were taken off, or broke off at some point. You won't be able to just replace the drive wheel, as whatever is available is ONLY sold in complete set of 4, at a typical price of $40 per set. The 1681E and 1661E are basically the same, with the 1661E listed as Lionel-Ives on bottom plate, and 8 spoke wheels.
