50cc vs 80cc

The actual law of physics will tell the truth. You first remove the head from the cylinder, seal off the valve ports, and fill the head space with fluid. Once the head is completely full, you dump the fluid into a measured jar, beaker, test tube, of some sort. This will give the actual dimension of the combustion head, from there, you measure the area above the piston when it is TDC.
Now you will have the actual true reading. If the piston is at flat with the top of the cylinder, then the liquid measurement is the true reading.
This is how the big boys with the big toys measure the heads on any hotrod engine.

Maximo

the standard way is to multiply
bore (how big is the piston) X stroke (how far does it move)

There are lots of ways to cheat when you think about it. On a 4 stroke it would seem that you would measure the full stroke but when you consider valve timing overlap you are not getting a full stroke of compression out of it, especially on race cars. On a two stroke your compression stroke does not really begin until the rings cover the ports but for marketing I am sure the full stoke is counted there as well. My mic set is has been history for decades so I have not indulged my couriosity on how many cc's of true compression I am getting out of my 50cc, and there are so many other factors involved anyway it is not that important to me.