Another simple idea. If it's not a real high torque situation, just slip a straight 1" shaft coupling on the splines and drill a hole thru and put in a bolt or pin.
I was looking at that, too. I don't have the exact measurement of the shaft other than 12 spline, 1". It will be high torque. I was thinking of pinning it to a collar, too, as suggested. I sent an email to the motor vendor, asking if they have a solution. No answer back, yet.