Every centrifugal clutch I have seen or heard of that is made for these engines is a tapered fit onto the shaft. There is no woodruff key used to prevent slipping on the shaft, and most will slip over time. Some even damage the shaft if left to slip and spin long enough.
The only way to remedy this is to make your own woodruff key keyway in the hub of the clutch, or some folks notch the hub and custom fit a key in such a fashion as to prevent the key from coming out on it's own. The hub is tempered very, very hard and to cut it typically requires a diamond bit or two with a Dremel tool.
Taking off from a dead stop will only stress the connection with the shaft and if not keyed, will lead to slipping on the shaft sooner than later.