Ian & Judy,
The coupling is a short (approx 6 cm.) steel sleeve that internally has splines. The bottom half is attached to the vertical drive shaft and held in place by a steel pin that is driven through both the fitting and the shaft. The top half of the fitting slides onto the splines on the motor shaft. Pretty simple really. If turning the prop turns the shaft and the fitting is intact at the top of the leg, you should be good to go. You can test the motor by running it briefly with the foot lowered to make sure that it's shaft is turning.
I recommended in an earlier posting that you add anti-seize compound to the motor spline / coupling spline to prevent future antics.