Regarding a repower. Your project is a "labor of love" based upon what you want rather than "practical" considerations. You have put an lot of time, effort, and money into it for that very good reason of getting just what you want. If after all of this, you think that when you do take her out, you will be nagged by the feeling that you should have put in new engines, then probably you should.
My brother in law undertook a similar restoration of a 43 Gulfstar Trawler that had been in the family since 1976. After he was through, about the only thing original was the hull. He replaced the Perkins 354's with 4 cylinder Cummins turbos. About the same hp, but a lot less weight. The boat gained about 4 kts and was much more responsive.
I like my Detroits but worry about parts and service in the future. Modern engines in the Gulfstar made her into a new old boat.
I wonder if your interior work has reached a point where the repower will damage the work you have done. Considering the accessibility problems of a motoryacht, I wonder if cutting an access hole in the cabin roof would be the best way to remove the old engines and install the new ones.
Beautiful project, keep us informed.
Good luck,