I am by means an expert on this matter and some have different opinions as to the actual rules but this is what I understand. The EPA wants new engine purchases to comply with the current Tier(emissions) requirements. So, if I buy a new engine for an old boat it is supposed to comply with the current regulations. Some in the industry have told me that is not accurate and that I should be able to purchase a new engine that was compliant when the boat was built. Non-compliant engines are still being built and sold. I was able to purchase a non-compliant engine by requesting a waiver from the EPA via the engine distributor. One of the conditions of the purchase was that the old engine has to be rendered unusable. In fact, I was not able to purchase the engine until the old engine had been removed from the boat, rendered unusable and then proof of same provided to the EPA for their approval. That's the best explanation I can offer.
The reasons for wanting to use a non-compliant engine were mostly dollars and sense. Mine is a 50 year old boat and I would have been forced to modify, (spend more money), to use a compliant (Tier 3) engine. The old engine was a Yanmar 6LPA and I replaced it with a Yanmar 6LP. The new is the same size as the old and did not require any modifications to the engine bed, wiring, plumbing, transmission, or propeller. It was a direct swap. At the end of the day it was still an expensive investment to make in an old boat but prior to the repower I had refitted the entire boat so now it is a brand new, old boat. Ha!