For the E-go, you would need to modify almost all of the electronics whether you go hub motor or with a new belt-motor. They have their whole system integrated and they really don't want it messed around. I've experimented with the Backfire/Melonboard/Lecdec board and replaced the belt-motor with a Single hub, but the braking is weak unless you can put a VESC in there and modify the braking curve. In the end, it will be easier and better building one yourself.
All the parts for building a complete board are available, you would just need to connect them. Check out http://runplayback.com/how-i-built-a-diy-eboard