The amount of thrust (forward force) you need in order to match a typical hub motor is something in the realm of 12 pounds to 20 pounds.
If you're talking about a motor built in to a 24 inch wheel, that should translate to 12 foot-pounds to 20 foot-pounds of torque. (because the radius of the wheel is one foot).
If you're talking about a motor with a friction drive system, driving a small friction-wheel, 2 inches in diameter, divide by 12 to find the foot pounds required but multiply by 12 to find the RPMs required.
if the small friction wheel is 1 inch in diameter, you'd divide by 24 to find the foot pounds required and multiply by 24 to find the RPMs required.
RPMs needed:
26" wheel at 10mph : wheel turns 129RPM
24" wheel at 10mph : wheel turns 140RPM
if you double the miles per hour, double the RPMs.