With enough time and money you can get whatever engine you want in there. I've seen a few Mailbus and Nautiques that have been Duramax swapped. There are a few options, direct swap, upgrade, replace or the do anything you want.
Direct - Marine Power still sells 5.7 repowers so you can pull the indmar and replace it with Marine Power. Though you may need to contact them to see if it will work with Zero Off
Upgrade - You can find a PCM dealer and move to the new Chevy direct injection engines. Overall should not have much issue fitting a Gen V Chevy small block in place of the old SBC
Replace - At the end of the day, a Chevy 5.7 is a Chevy 5.7. Most shops want the simple and least labor routes, so they don't usually mention the replace route. You can get a Vortec 5.7 from a place like Michigan Motors and have the shop move all the other components to the new long block
Do anything - again with enough time and money you can get whatever engine you want in there