Ya know, I think if you could put the right size rack on the back, you could mount a little Go-ped motor on it. You'd need radically different gearing, of course. And I would even sneak in slightly longer handle bars.
It would be pretty much a "suicide-ride" anyway, no matter whatcha do. But at least a person could claim they have the world's smallest functioning motorized bike.
You could maybe even train squirrels to ride 'em and start your own circus. 'Course, you have to build one for the elephant too; he'd get jealous.
Seriously though: I'm pretty sure that little bike was a custom-made one-off, but I'm still tempted to inquire if maybe anybody knows if there's a plan to sell these tiny things? Being that they made that monkey bike, I never can tell what companies will sell these days.