Just to throw in my 2 cents, thought I would share my recent repower experience. I repowered my 83' Islander Bahama 30 this year with a Beta 16 after a good deal of research on models from the major diesel manufacturers.
Both Yanmar and Beta (Kubota based) have models that have mounting dimensions pretty close to the Volvo MD-7A. The Yanmar is a great engine too, but I chose the Beta because for a little less money I got more Hp (16 vs. 14) and I really love the serviceability of the Beta. I've been able to go to the Kubota tractor dealer and purchase oil filters, fuel
filters etc..., and the built-in oil pump
makes oil changes a snap. I considered also going with a 20Hp from both Yanmar and Beta, but after talking to Beta they convinced me that the 16 Hp model was more than adequate. I don't regret my choice as I have plenty of power now and the engine is very economical to run. I think the real advantage of going to the 20Hp models is they may run a little smoother as they have 3 cylinders vs. 2 cylinders.
The install did involve raising the engine mounts
and a new shaft was required as well. It also required a new prop (needed one anyway) as the Volvo turns the prop CCW in the forward gear, the Beta and most others are CW.
You'll find the new diesels are almost half the weight of the Volvo for nearly the same Hp. The Beta runs very clean and diesel fumes seem nonexistent. However, I do miss the Volvo's big flywheel when at idle as it did a good job of smoothing things out. The Beta is smoother when at power though, and I just don't leave it at power for long at idle.
Good luck if and when you decide to do your repower.