Since the market for under-35 footers went south about 10 years ago, I can't think of a single
Good Old Boat (by definition older than 1984 or so which was when the Atomic 4 ceased to be made) for which a diesel repower
would make any kind of economic sense in terms of resale value.
If you had a duff A4, were 40 years old and wanted to switch that 34 footer from daysailing to "light offshore" or "extended coastal motorsailing", in the context of a 20 year ownership
of the same well-maintained boat, yes, I would consider dropping in the Beta replacement, retanking, reshafting and repropping, and would think myself lucky and able to do 90% of my own labour if I got away under $12K.
You can get some nice, entire, working and plaid-cushioned, freshwater-only boats from the '70s-'80s in that size range for $12K these days.
I base my opinion on an ongoing but nearly finished complete replacement of a 1988 Westerbeke
52HP with a new Beta 60 in a larger boat. It has made sense to do so and has come in more or less at budget
, but it's neither been fast nor cheap
; swapping out an A4 wouldn't be, either.