As I recall, Ammarine was a marinizer of John Deere engines, like Lugger, before JD started selling marine engines themselves. Lugger survived that event, but Ammarine didn't.
So, most core diesel parts can be sourced from John Deere, but those rarely go bad. Common marinization parts like impellers and even heat exchangers can be sourced from the original supplier, like Johnson for raw water pumps (just a guess, don't know if it was a Johnson). Where you will find it difficult is if a major casting like the exhaust manifold goes bad. I guess you can always see if Lugger made an equivalent one for their engines and replace it with one of those.