1973 was about the period the engine was moved from the centre of the boat to under the steps, what I write here is for the centre location only.
There are no diesels of sufficient power (about 22-25HP seems the consensus from the folk I have spoken to) currently in production that fit in the space, a couple have been made in the past by Westerbeke and Universal which may be available used. The closest new engine fit I could find was by Beta Marine, which fits on the same mounts but rises a couple of inches higher than the engine cover allows, which means some carpentry after which that seat at the cabin table will be a couple of inches higher than the rest. Beta Marine told me that I would need a new shaft, stuffing box, prop and cutlass bearing, which would be a $7000+ project if I did all the work myself.
Rebuilt A4s are fairly widely available as replacements, which is what I ended up going with. On the east coast,
www.moyermarine.com is the best known source, plus I have spoken to other small town mechanics running rebuild businesses. Many want an old engine to turn in. There are other sources on the west coast doing the same thing, so I would look at that option carefully if $3000-$4000 looks more appealing.
Either way it is a long, painful process... Good luck.
Gareth
Freyja E35 #241 1972
Aquaholic