From looking at your picture, they look like standard O Gauge wheelsets.
Agreed. A regular postwar Lionel wheelset measures around 5/8" across the tire, which is probably close enough for gummint work.
However, beside the one wheelset needed, you will also need two bushings (cylinder-like contraptions that allow the axle to turn freely) and two pieces of some firm material (that still has a little spring) to squeeze into each journal to hold the bushings firmly in place within the journal.
I have had good success using Plastruct round tubing as bushings. As for the locks...well, if you're not planning to change the axles, a little ACC on the outsides of the bushings should hold 'em in place.