Ford Lehmans have been put in a LOT of
boats for more than 20 years. It would be hard to think any good desiel
mechanic couldn't do the job. They are not that special. Finding a good mechanic is mostly asking around and checking
references.
You really want one within an hour of your
boat. Most will charge travel time if they are any good. The best mechanics I've ever used did not
work for a marina but were independant. Some of the worst ones I know of
work for a marina. The "Great Lakes area" is pretty big.