Very nice job. If it were me i'd let the front truck be the dummy and consider powering the rear one which from the photos i've seen appears to be of a more traditional configuration with equal diameter wheels. Depending on the wheelbase needed i'd consider the power trucks used by Lionel on their RS-3's or the ones used by KLine on their S-2's. If either of those were of the correct wheelbase you could simply grind or sand the side existing plastic side frames flat, and then using scraps of plastic and brass build up the detail of the side frames to replicate those on the McKeen cars on the flat surface you just created. If neither of those offered the correct wheelbase i'd search for an existing truck of the right wheelbase(probably from a passenger car with plastic side frames), and use the same treatment as above for side frame detail; then get the keyhole style can motor mount and motor assy. from any small "China Drive" loco; then send the truck and motor assy. to Frank Timko for gear installation on the axle. A piece of sheet brass with a key hole cut in it and then screwed to the floor will serve as the mounting plate for the vertical can motor. I've scratch built a few projects using both these techniques. The drive of any project of this sort is always a problem, but why compound it by making that unique front truck the powered one, and why try to "re-invent the wheel", so to speak, if you can use and existing drive unit? Finally, i've tried the Magic Carpet and similar HO intended drive units and found they lacked the power needed to handle the weight of an O gauge loco.
Good luck and feel free to contact me by email if you have any more questions.
I just checked Fleabay - search motors under O Scale trains, pages 1 & 2 have some possibilities --this is what i'm thinking of http://www.ebay.com/itm/LIONEL...;hash=item232ea0da92 - Found on the second page.
Hope this reference doesn't violate any Forum rules