I've concluded that it's probably not worth buying and installing a used engine, even after a full renovation - I think the total cost (including the value of your labor, and the hassle factor) will add up to about the same cost as buying a new replacement engine, with a warranty. I researched recently how much used engines go for - I don't think I'd be able to trust one of them, and would rather pay the extra for a new Beta marine replacement.
The reason I was pricing used replacement engines was that a guy at our club was parting out a Catalina 30 he bought from a resigned member who'd basically abandoned his boat - I wanted to put in an offer for the engine and transmission as they were exactly the same models as in my boat. I got the combo (and the old Racor filter unit) for $650, which is far, far less than 'working, but unrenovated' engines are advertised for on line (before you also pay to ship them to you). I planned to leave the engine at a local shop to refurbish at their leisure, after which I'd swap it for mine, and then use mine for parts. But, after getting it into my garage at home I realized it has 4 mounts instead of the 3 mine has. I don't want to rebuild my engine bay, so now I have a nearly-identical engine in my garage.
But, it's still very useful: every time I need to do maintenance or repair on my boat's engine, I can do a dummy run at the same task in comfort at home, and then go (with all the necessary tools and parts) and do the same task on my boat's engine, in the cramped awkward space on the boat. Also, the spare engine may eventually be useful for parts - especially the really expensive non-Kubota bits, like the exhaust manifold.