Sell the Johnson, use the money to fix the A4 and go sailing.
I lean towards JSM's advice. But with the current covid boating restrictions, maybe now is the perfect time to start a big project, since we cant go sailing anyway. I have often pondered about going to electric propulsion if my diesel died. I love the concept.
Especially in the summer on the Chesapeake, about 50% of the time is spent motoring if you are wanting to move each day. With that range consideration, a hybrid would be intriguing. No concerns about charging the fewer batteries required, power for autopilot, refrigeration, air conditioning(be still my heart!). It seems using off the shelf parts for a hybrid would cost about as much as a new 16hp diesel. Having personally fitted a WWII surplus electric motor to a VW bug, I know it can be done cheaper by scrounging. Again, sailing vs working on the boat. A true hybrid setup would be a tight fit in a 30ft boat. Instead of a true hybrid, a portable generator would extend range some, but couldn't likely keep up with continuous motoring. That might extend the range enough.
My other concern is would it have enough power to keep my family off a lee shore? On the shallow Chesapeake, thunderstorms are a daily occurrence in the dog days of summer.
Please keep us posted, I hope to see you out there soon!