it's kinda hard to measure the stroke because the plug hole isn't straight on and you can't remove the head with 1 piece jug. The engine tage even if it doesnt say ABC orD will say 47 or 49cc - 49 is the 44mm stroke D crank. The quickest check if you suspect it's been maybe swapped, if it's a square C/D cylinder, look up into the exhaust port at BDC and if the piston hangs over the bottom by ~1mm it's probably the shorter crank.
It's awkward trying to measure stroke outside the bike with the jug off. The other telltale if you have the crank outta the cases is the location of the bigend bearing - on the D the bigend rod lines up right next to the taper on the cheeks, the ABC 42mm crank the bigend is close to the center of the cheeks with some space before they taper off. Search for crank diffs or look in the wiki on sachs motor differences for photos