This design looks like it uses conventional bicycle parts to drive the rear wheels independently. I think the plans are $17 from their website.
Then you might be able to adapt a kit like this to it:
24V 250W ELECTRIC MOTORIZED E BIKE CONVERSION KIT | eBay
Or if you use conventional bicycle forks for your front wheels you can use hub motor wheels in the front.
EZ-Glide Electric Bike kit for 26 in Bicycle | eBay