This is the only way I can think of:

Step 1: take out a spark plug
Step 2: put in a length of dowel lightly in to the hole so that it sits on top of the piston
Step 3: wind the engine over by hand until the dowel goes as low as it can before it starts to come up again.

Step 4: put a mark on the stick with a pen
Step 5: wind the engine over again so the dowel comes up.

Step 6: mark the dowel when it won't go any higher again.

Step 7: measure the distance between the two marks

3S stroke

(2.0): 86mm
5S stroke

(2.2): 87.1mm

Good luck though, not much difference between the two!