Yes, three immediately come to mind: 1) The tolerances between a vertical pinion gear and a horizontal ring gear are quite critical; do you have the tools to properly align these and are you familiar with the use of Prussian Blue and lapping compound? 2) The vertical play becomes more important with a leading truck opposed to a trailing one in the prevention of derailments, are you planning on building in a means of adjustment to get the amount of play exact? 3) The entire motor/drive/truck assembly will need to be mounted in a way to swing to horizontal tolerances of 1/32" or less (while holding this tolerance over all vertical motion) to prevent derailments (esp. through turnouts and while reversing). Do you have the tools to accomplish this?
I simply don't understand why you seem to want to further complicate this project by powering the front truck!?!?! if the reason is to adhere to the prototype then power it with a miniature gasoline engine!