I've worked on the 'power supply' and the trucks mechanisms. As mentioned before I switched to 36V-voltage provided by 3 automotive batteries wired in series (each has 55Ah).
Here's the additional sub-frame for holding the batteries - on top of the unit's main frame:
Since there is need for 12V (lights, horns etc.), too, I've used the space of the 'tank' below the frame to place a small 12V battery:
Each truck is powered by a 36V, 350W-PM-motor - connected via chains and gear reduction to the outer axle.
The three axles of each truck are connected by chains: