Believe I'd fix the Perkins and call it good.
The Volvo-Velvet Drive connection would be a big deal; if it's not a perfect match, then you're talking new gear, maybe shaft... plus all that extra work to do the swap, on top of all the work to change the engine...
When we had our Mainship III with 220-hp DD 8.2T.... the boat wasn't a speed demon.
That Perkins would probably one last another 72 years, though...
-Chris