Vetus, Nanni, Beta, Yanmar Volvo are the big ones. We repowered with Beta, probably the most thoughtful marinization package of the group and they really specialize in this market, second would be Yanmar, for out of the box done. You could Try Freeport Diesel in Maine for local recommendations. It's a bunch of Maine Maritime grads who sailed commercial or still do, most worked for me at one time, very good guys.
Also know that not all of these small diesels are emissions-legal in all states. I don't know what the status is in MA, but in the state of denial (CA) it would not be legal to repower with a 1GM10. Don't know about Beta, etc.