The Colker bike would be a good candidate for a motor for sure. You might want to use a 4 stroke though for the torque to get them up to speed, but once you do can you say monster truck?
Because of the interest of the unicyclists there have been some huge advancements over the Colker rims which were heavy and the tires didn't have much of a tread, and were heavy which made them kind of hard to get up and going especially because the stock bike used a really short crank arm for some reason? Anyway unicycle.com stocks the better quality rims and tires. The uni guys like them for the obvious reason that they roll out more than a 20" one does. They adopted the 29" when it came out and I guess if someone made a 60" wheel they would use that too? Unicycle.com :: Unicycle.com Home Page
Quite a few custom framebuilders are messing around with the concept and here are a few examples. The second one recently won best of show at a custom frame builders event in Denver. Yes, it is Ti.
The coolest one I have seen though had a custom 130mm front hub with dual disc brake but I can't seem to dig that up from the files right now. Also there was one for sale real recently on fleabay that was made by a guy in OZ so you boys south of the equator should be all over it. The way I see it after having ridden that wheel size is that it would be much better with some assist and I know it is on my want list to build one up some day.