if you are pulling your boat onto shore, you also need to be concerned with the entire boat being stolen. As far as rhe motor is concerned, a lock through the clamping handles or the type that covers them comes to mind. you could also run a cable fron one of the stern carrying handles to the motor.
To prevent theft of the boat you need to chain it to a tree. If you could also attach tin cans that would rsttle when the chain is moved, that might alert you. When travelling, I secure my luggage with an alarmable cable lock. If it is moved a piercing alarm goes off, something like a car alarm. Could work for you?