I have an O'Day 30 with a seized Yanmar YSM12. I am in the process of discovering more than I wanted to know about the cost of a replacement engine. I am not familiar with the center cockpit type but mine is quite simple with access behind the companion way steps. It took about an hour to disconnect everything including the starter and alternator. The motor would have already been out but I discovered that the dripless seals are not so dripless when you slide the motor and transmission away. :-). I wonder if schooner bay has YSM12s laying around.
I changed the oil and water pump annually. Not sure what happened.