Honestly, Rip the head off and measure it with vernier calipers.
Example 1. Just yesterday I pulled the head and barrel off a Lifan 1p54fmi. Now, I thought that this engine was supposed to be a `square' 54x54. Turns out that it had a 52mm piston in it????
The chinese are pumping out so many of these engines that I think that when they run out of the correct cylinders, they just throw whatever piston and barrel combo they can find laying around onto the things to get them out the door.
Only way to be sure is to pull the head and check.