Very good to see you making the attempt to create a reasonable representation of the unique drive truck of the McKeen car. Just plopping a body on a diesel drive is really an visually unsatisfying compromise.
I'll reinforce the the Old Goat's suggestion that you look at the NWSL and Q-car for powering ideas. From NWSL you could get a Magic Carpet drive unit that might be built into that same truck w/o taking up excess space that would also fit under the floor neatly. From Q-car you might be able to get a geared axle transmission unit with can motor that you might ber able to build into the truck. Whether to put these into the power truck vs. the trailing is another decision to be pondered.