I should of asked at what speed ... that determine the torque...
They can't handle more than 26 gear-inches of torque (human legs). And they weigh 11lbs. The maximum torque before it starts slipping is 130Nm or 96 LbFT . One wheel revolution to one crank revolution is the lowest gear possible.
I think that unless you crawl up a hill much slower than walking, any motor will make it slip.