RE: The Wonderful World of Half-tracks!
As far as a working chassis goes, I've seen builds where they used 1/25 Panther tracks (and drilled holes in the roadwheels to mimic what you see on a halftrack) that worked very well. There was even an RSO build where he used those components and dummied-up the drive sprockets so they didn't look so much like ones from a Panther.
For a motor unit, I've always wondered if the forward-and-reverse motor that comes with the really basic Tamiya 1/16 King Tiger would work. It wouldn't have variable speed control, but since it was made to motivate a KT, it ought to have enough oomph to move anything you could build around it (think Famo).