Once you find the theoretical diameter by measuring the circumference of the irregular sized hole, the rest of the calculation is treated like a circle. You didn’t mention the gasket thickness, but with a 4.237” diameter hole and a 0.041” thick gasket, head gasket cc’s will be 9.47cc’s.
Here’s the calculation.
(4.237) X (4.237) X (0.041") X (12.87) = 9.47cc