Aircraft engines were hemispherical way back. The fuel mix swirls in them and the twin redundant ignition systems ignite the spark plugs at 180 degrees from each plug center inside the cylinder. The flathead has more than a swirl in the chamber due to it's design but the fuel mixture does get burned efficiently. That's one of the reasons it doesn't need as much advance as an overhead valve engine does.
The really large engines have two magnetos and two distributors. The magnetos provide the high tension and the distributors clock it to the correct cylinders. The big Wright R-4360 has 28 cylinders in 4 rows of 7 cylinders each. That makes for 56 spark plugs to fire for each double rotation of the crank. The really complex one was the Chrysler Multibank engine in one of the M4 Sherman tank models. It had five 6-cylinder engine blocks joined into one casting. That's 30 cylinders to fire in two rotations. It was an amazing design. You can imagine the 5 ignition systems on that baby.
https://en.wikipedia.org/wiki/Chrysler_A57_multibank