Rebuild or Replace
I sure wouldn''t toss the engine just yet and rebuilt 4-154''s are available and worthwhile to look at should you really need a new engine. The 4-154 is a nice reliable engine.
You may not need even a total rebuild either. Get a good mechanic to look at it. If it needs a full rebuild then pull it, and ship it to a rebuild shop. Trans-Atlantic Diesel, White Marsh, VA is one of the best on the east coast. You generally will save on a rebuild if you take the engine out and send it to a shop. They can do it for one third the price in a shop vs. what a mechanic on board your boat will charge you to do in place. It may be a PITA to pull it out but worth the effort.