You could try contacting John Skewes of Cellar Marine, Porthallow, 01326 280214. He is a main agent for Yanmar engines. You will probably find him at the Southampton Boat Show in September. But in the meantime, try phoning him.
I had a Yanmar 10 in my last boat, and he completely overhauled the engine when the head gasket went, allowing water into the sump.