Ok my turn!. If in regards we are talking about the gaskets at the base of the cylinder where it bolts to the crankcase. If you increase the thickness of the gaskets you will indeed increase thevolume inside the cylinder. However thedisplacement will remain the same as you have just increased the volume of the cylinder thereby decreasing the compression ratio plus the piston to head clearance. Same idea when you use shims under the head between the cylinder. Also changing the compression ratio either up or down will change the static timing for the ignition which can also be done by using different octane fuel inverserly, higher octane retards the flame front and lower octane increases the speed of the flame front. Higher octane fuel burns slower than lower octane, this is a generic statement just to get my point accross. Octane also changes the chance of detonition (spark knock). Enough about fuel back to displacement.
Shims are used to increase or decrease compression AND head to piston clearance. They do this by increasing or decreasing displacement and distance between the top of the head and the top of the pistion.
I used to race go karts in Germany with 100cc rotary valve engines where the piston to head clearance was only 2 to 3mm and getting about 26.5 to 28 HP with tuned pipes. Engines were made by Pirelli and Komet, 2 and 3 port. So I do have quite a bit of experience in 2 cycle engines. I am a Master Diesel Engine Tech and build and repair engines from 5.9L up to 16L. Now we are getting into liner protrusion, head clearance, gasket thickness etc....

Bore x stroke = displacement. Bore x stroke does not equal cylinder volume unless the piston touchs the head and the head is flat, not domed.