For your information, according to my 1969 Bertram specs & pricing for the 38 Salon Cruiser, the diesel option was the GM 8V-53 @ 283 HP. Interestingly, the 325 Mercruiser (427 c.i.) was the standard power. The gas boat, BTW, cost a tick over $50K while the oil-burner had almost a $20K premium.
You paid less than that, right?
Regarding that $60K replacement figure mentioned up-post...well, we rebuild modern 8V-92s for less than that around here.
While production boatbuilders are smart enough to simplify things when offering two engine options, i.e., I'll wager that the reverse gear was the same for both engine packages, and the stringer width was the same, etc,etc....I believe your short path, with the least pain & aggravation, is to unbutton the bad motor--and God knows there are 53-series parts aplenty (Check 'Boats and Harbors'--that yellow rag)--and have at it.
It's not Rocket Surgery, you know.