I remember seeing pictures of a guy who had attached a gear head 90V DC motor in place of the crank. Looked like a really slick arrangement. I haven't been able to find the images though I'm pretty sure it was in this forum or on Bob Warfield's excellent CNCCookbook site.
To estimate the ft-lbs needed attach a lever the radius of the planned sprocket to the shaft. Attach a spring balance to the end of the lever. Lift up on the spring balance until the lever moves. The balance gives you the "lbs" part and the lever length gives you the "ft" part of the ft-lbs measurement you want. I believe the Machinery Handbook talks about this if you want to read more about it.