I have a 1979 Seafarer 30. It has a Yanmar YSM12 diesel. I am in the process of rebuilding the engine. I have been able to find the parts I need from Yanmar. Yours may be the YSM 8, which is very similiar to the YSM12. I have a parts manual and rebuild manual for the engine that came with the boat. I would assume that you could still buy these books from Yanmar.
There has in the past been a pretty active Seafarer discussion community hosted by Sailnet. I'm not sure how the group is carrying on since the email list appears to have been shutdown.
I have only seen two Seafarers first hand. They don't seem to very common, but, the Seafarer 30 fits our sailing needs perfectly. I hope that I am successful with the engine rebuild so that I don't end up in a engine repowering process.