Your location does not make this an easy answer.
Just beware of the "guy" who says that he has a direct diesel replacement for the A-4. While it may have the same footprint, keep in mind you will need a new fueltank, new fuel
lines run and in some cases an enlarged exhaust
I work on A LOT of A-4's. If I were you I would go to www.moyermarine.com
and get on the forums
You say the freshwater jacket failed, how did you determine that, where is it leaking in?