It's going to be a bigger project than just switching the wheels. There are a few options out there, but you'll need to factor a few things also:
Wheel clearance - once you choose a wheel you'll need to test fit it to see if you're going to get wheel bite.
Gear ratio - The E-go was designed for for their specific wheel and wheel diameter. If you change just the wheel your board probably wont have the torque to do much.
You'll need to change the pulley size and belt size. At that point you may also need to change the motor pulley too.
Adapability - There are probably just a few choices for all terrain wheels. XT, Steadfast, MBS, Enertion, Alien, and my wheels.
http://xtwheelz.com/index.php?main_page=product_info&cPath=65&products_id=180
http://www.jacksboardhouse.com/Steadfast_Off_Road_Longboard_Wheels_p/ofrod.htm
http://shop.mbs.com/accessories-34/mountainboard-wheels.html
http://www.enertionboards.com/electric-skateboard-parts/all-terrain-5-inch-wheel-assembly/
http://endless-sphere.com/forums/viewtopic.php?f=35&t=55843
http://endless-sphere.com/forums/viewtopic.php?f=35&t=60449
http://endless-sphere.com/forums/viewtopic.php?f=35&t=59259