The issue is lack of the EPA label...
You may or may not be able to get the engine
maker to document that the engine meets EPA as is.
If it doesn't meet EPA, then you have to find out what modifications are needed.
That might turn out cheaper to dismount the engine so its no longer part of the boat
and look for a different engine after importing.
And don't get the idea of putting it back... while chances of getting caught might not be high, the fines are.
Or just keep the boat registered as it is registered now.