Manifold for Yanmar 3HMF

[FruiVita]

I am looking for an exhaust manifold for a Yanmar 3HMF marine engine, serial number 00186, that is currently in my sail boat. I was told by a mechanic that it is severely corroded but that "they have very thick walls and it could be years before it rusts through". Not wanting to risk having the thing fail I'm thinking of replaceing it as a precaution. The boat is a 1985 and it is very likely the engine - including the manifold - is original. Any suggestions on how to come by this part? It appears the engine is no longer in production.