An acceptable hub for 170mm spacing and 6bolt mount with sealed bearings (the OR8 is an example) would cost 70-90$.
I would go with something on these lines. found this one on the Bay...http://www.ebay.com/itm/SUN-RINGLE-ABBAH-HUB-WITH-12MM-AXLE-FAT-BIKE-FAT-TIRE-/221570956221?pt=US_Hubs&hash=item3396a87bbd,..I know it is half the price of the whole bike, but wheels are the most important part of a bike, and with that hub the bike will be another one compared to the stock. the BB replacement with a quality one, alloy brake levers, and secondarily, a precision shifter/derailleur, would complete the feeling change from a toy-kid bike to a rolling serious machine, ready to be motorized.
The XD for drumbrake fit perfectly _(in the low end power range), I suspect that drum brake rear hubmotors, could be better adapted to a fat dropout using the threads of the drum to screw on a 6bolt 44pcb adapter for the disc. Using some spacers both side you'll be pretty able to get an usable motor, that's exactly what I have with the XD. A 9-10 speed freewheel is allowed, too
. The axle should be anyway strong, but i guess any hubmotor has a stronger axle than the stock one :wink: .
Ah, Obviously the drum hub with the adapter could be used for brake rotors but is useless for left driving purposes....
Cheers