One of the better electric assist options I have seen for Big Dummy/Xtracycle bikes is a mid drive unit like the StokeMonkey. It was originally available by CleverCycles, but hasn't been available for a few years.
ebikes.ca indicates that it's supposed to have a new revised version but who knows when...http://www.ebikes.ca/store/store_stokemonkey.php
You could DIY by making a mid drive bracket for the frame, and getting an electric rear hub with threaded single speed freewheel, and drive it with a LH crank-arm/spider, some batteries & a controller...