Yanmar 12 Seafarer 30
Are you sure it''s a YSL12? I''ve heard of YSE, YSB, YSM 12 (early, middle, and later models, don''t ask me why). I have a 1978 YSB12 that got a partial rebuild 2 years ago (new gaskets, minor head work, etc). It starts right up, even when the air and water are both at or near freezing.
Also I''ve been told by several mechanics the old Yanmars were very sturdily built. Just make sure you keep it cool (raw water cooling, so keep an eye on it; I have a bilge blower for giving it fresh air too), change the oil and clean its little oil "screen" (since it''s not really a filter), and clean fuel.
marinedieseldirect.com has lots of information on Yanmar engines. Get the service and parts manuals; the service manual isn''t well-translated, but the illustrated parts breakdowns in the parts manual are invaluable.