This is the link to American Diesel. The owner, Bob Smith, was president of the Lehman division of Ford. American Diesel Corporation
They can put you in touch with companies that can give you a price for a rebuild of the Lehman. Unless you do it yourself, I'm guessing that it won't be that much less than a new Yanmar. The real question is, do you want to put a new engine in a '72 boat or spend less with the rebuild.
Lehman is a great engine, but even one that has been babied, will need major work @ 100,000 hours.