There are lots of sister ships around with diesel conversions. Hopefully some owners can chime in with comments.
I recall pix that show a raised seat (by an inch or so) over the new engine, because nothing sits quite as low in the boat as the original A4 flathead gas engine. (BTW, the Catalina 30 built in the 70's has the same problem due to the same settee location for their engine.)
Do some measuring of position of the mounts now and the overhead room. Check out engines from Yanmar and Betamarine.
Our recent upgrade to a new Beta did involve a wood engine attachment mockup, but then we knew that we had enough overhead room anyway.
D/L the drawing and do some sketching and pondering....
https://www.betamarinenc.com/engine/beta-20/
Look at the second picture in this blog entry to see the engine mockup.
http://www.ericsonyachts.org/infoexchange/entry.php?617-Old-engine-out-planning-for-the-new-one
Personally, I would opt for a slightly smaller engine like a 20 hp, just to minimize 'fit' problems. Note that these engines have an optional shallow pan and drain attachment.
I'm certainly no expert but do believe that modern current-generation diesels do produce more useable hp at the prop than did the ones in the 70's/80's.
Modern horses might be Clydesdales..... ?
Note that your almost-mid-boat location will call for some creative exhaust piping design as well, no matter what brand you choose.
Best of luck!
