[ngoma]

How did you try to crank it? 27mm socket wrench on the crankshaft bolt? Or did you try to crank it by the camshaft bolt? That can damage the timing belt, which leads to more, worse engine damages.

What did it sound like the last time it ran?

Were you already experiencing runaway problems before?

Beware: even "great engine builders" can get into trouble with these engines if they do not follow some of the (admittedly uncommon) crankshaft pulley tightening and timing procedures. Because these procedures are not common and require special tools, the otherwise "great engine builders" think they can substitute the common practices they are more familiar with that always produced satisfactory results in the past. Doesn't work for this one, unfortunately.

My Own Opinion: Highly doubtful that one winter season in a maritime environment could cause enough internal corrosion to seize the engine.