Although it is possible, ABC types of engines do not normally sieze up, it is in their design. But I have seen that happen twice to myself in the past. With a ABC engine if the engine happens to stop with the piston up neat the top of the cylinder and there is a lack of oil at that moment, it could get stuck. With ABC engines the cylinder has a taper to it that causes a pinch effect at the top of the cylinder. The piston has to be squeezed through this pinch when the engine is cold (which is why you aren't supposed to flip the engine a lot when cold). But when the engine heats up the cylinder expands and the piston fit is just about perfect. But if you stop the engine and there is a lack of oil at that moment and the piston is in the pinch zone when the engine stopped, then when the engine cools of the taper comes back and the piston gets squeezed and it is stuck.
I suggest heating up the engine good, like it is running, in the oven and that should get the cylinder to expand enough to let you turn the engine over. Say set the oven to 250 degrees F or therabouts. Using some penetrating oil in through the glow plug hole is a good idea too. Don't try to force it too hard as you could bend the connecting rod. Remove any plastic parts if there are on the engine, such as the carburetor when you go to heat it up.