I would agree with marinheiro. I replaced a 35 yr old engine, that was working fine. It is all the ancillary bits that fail / fall apart. For this prospect engine of yours, if it starts and runs fine, well, it starts and runs fine. I would be most interested if you could get it up to full revs under load (i,e. motoring the boat). My old one would shake the boat to bits above mid revs.
You would probably get more info out of a thorough visual inspection of the ancillary bits, and if possible, an understanding of the maintenance history. And I don't mean oil changes by that, but engine moun