How long did the engine sit idle without being turned? Whenever an engine stops, at least a few valves will be open to the atmosphere. This allows humidity to cycle in and out of one or more cylinders and promote rusting. Depending on how humid the climate happens to be, a cylinder can rust heavily enough to lock the engine up in a matter of a few months in some cases.

I'd suggest pulling the cylinder head first, and inspect the cylinders for rust accumulation around the tops of the pistons. Once the pistons are exposed, a good soaking with penetrating oil and some energetic massage with a block of hardwood and a heavy mallet might get things moving.
Jerry

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