Bore x bore x stroke x .7854 = displacement. If you measure in inches, that will give you cubic inches. If you want cc's, divide that result by .061.

For a quickie "cheat" measure on an assembled engine, you can often stick a pencil in the exhaust port and over against the far cylinder wall. Rotate the engine so the piston puts a crimp mark on the pencil. That will give you a rough bore dimension. Then if the spark plug hole gives you a straight enough shot, you can insert a pencil and mark it at the top and bottom of the piston stroke, measure between the marks and that will give you the stroke.

AV8TOR