If you want to be absolutely sure about the engine, pull the head, pull the cylinder and see if it is a 8 hole cylinder and crankcase. Nothing is 100 % sure in life, but a 10 mm spark plug head always goes with the 8 hole cylinder. I have never seen it in a different way (but that doesn't mean that it is a fact).
Wrong again, thanks to the people who pointed out that this info is indeed not 100 % correct. It seems to be possible to find a 12 mm spark plug hole head on a 8-hole cylinder.
Thanks for sharing the info,
Jensen