Mike, 640-9879-625 is the part number for the SD70/ES44 wheels.
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Chuck... Unfortunately, the only numbers I was able to locate were for the current U33 scale-wheeled power trucks:
6408186770 / FRONT | 6-WHEEL POWERED TRUCK / A-F U-33C / 'SCALE WHEELS' |
6408186772 / REAR | 6-WHEEL POWERED TRUCK / A-F U-33C / 'SCALE WHEELS' |
which are, of course, listed as "Unavailable" at 60 bucks each.
And of course, Lionel has a parts breakdown for the Flyer wheeled power trucks, but not the scale wheeled trucks.
The only other option I see is to use the original scale 6408142625 conversion wheels and also change out both worm gears. I checked, it's different from the current U33 worm part numbers:
6408142520 | DRIVE SHAFT W/ WORM GEAR / A-F U-33C / 2009 | 14 | $12.00 |
Rusty