I started by putting down all the parts of the Lionel S-2 Dummy 3-rail locomotive.
The original Lionel model has no motor, no gears, nothing. Only a dummy engine like a "wagon".
I started using Atlas SW 3-rail trucks. I've swaped the 3-rail wheels against 2-rail wheels from NWSL and I added pick-ups.
I put off the EMD side trucks and install the "Blunt" ones from Lionel.
The Atlas SW truck "as bought"
Underneath, you just have to unscrew the 3rd rail pick-up and put off its wire.
4 screws to put off and the truck is easy to modify into 2-rail FS.
Here, you can compare the Lionel 3-rail wheels and the 2-rail from NWSL. There is just a very very little difference
The most difficult part of the motorization is done!
The video I've still shown on the "What Alco do you have?" thread