Richard, there is some discussion re engines in the SS27/Parker 27; 275; 285 area of this forum under the title 'Water Heating' but there is a more comprehensive selection in the old forum area under old pages 3, 4 and 7. I suggest you have a good look there.
I fitted a Lombardini 13 hp (fresh water cooled)to 'Elsa' (26 no 103) but others have more usually used the Yanmar 10 or the Beta 13.
Hope this helps.
Chris.