If the value of the boat has 'bout bottomed out, and you just want a way to be under way...
That smaller diesel will probably get it up to 7 kts, i.e. displacement speeds.
Many years ago I talked to the owner of a classic Tolly 34 (fiberglass, but an early one) whose boat had been converted to a pair of 50 hp Perkins diesels.
He said it would do an economical 8 kts. Evidently a prior owner had a lunched V8 on one side and chose to repower with the smaller diesels. It's been a while, and perhaps both original gas engines were shot for some reason, but I just do not recall the details.
Owner seemed quite content with it, and the boat was in good condition otherwise.
As to where to post your question, well, most of us here are effectively driving a "trawler with bridge clearance problems" half the time anyway!
I would also advise that you try to find a power boat forum where someone will know about the structure of your hull and may have suggestions for your scope of work... and then will try to talk you out of pursuing it!
ps: be sure to factor in a prop change and changing the fuel system over.