When you are pedalling the clutch is in and the chain won't lock up when the engine seizes. Or, on a non-motored bike, you probably won't come to grief.
Believe me if you have done thousands of miles on these motors the day comes when the engine seizes and you probably can't get the clutch in quick enough because the back wheel is locked and skidding outwards to the right and you are fighting to stay on it. It's all over in a couple of seconds.
I have had this happen although only at 15 mph and I ended up on my face in the ditch with the hot engine burning the flesh off my legs. If it had happened on the freeway 2 minutes earlier I'd have been dead. If you want speed then get a freewheel hub of some sort so you live to tell the tale of the engine seizing. At 40mph a wheel lock up would throw you off in a split second. Cycling Australia states that hitting a labrador dog at 35mph is usually a fatal accident on a bicycle. Just a warning from a cautious old cyclist.