Just a couple.
You'll probably hear from loads of people that you should just rebuild the A4. Cheap and not that hard. About 35 years ago I was in a similar situation with my E27 and decided to put in a brand spanking new Perkins M20 diesel. Everyone thought I was crazy and that I'd never get 'my money out of it' but I had 30 years of flawless running before I sold the boat so figure it was worth every penny. But you should probably consider how many more years you want to own the boat when you are making this decision.
A couple of other things to consider when looking at this project:
You won't be just changing an engine and a prop. More torque out of the engine may mean a bigger shaft as well. I had to go from a 3/4 inch shaft to a 1". Which also meant new stuffing box and strut bearing. You also normally go to a larger diameter prop so need to make sure you have clearance between the prop and hull.
It also means a new fuel tank. Or if you are lucky, just a modification to your existing one. Diesels require a return line to the fuel tank. I was able to empty my gas tank, drill a hole in the top for a return line and got off easy. One BIG plus with the diesel is that same fuel tank will have 4 times the range it did as a gas tank. My A4 burned a gallon an hour at cruising. My Perkins a quart.
The other big thing is don't believe any advertising about a 'drop in replacement' for an A4. They don't exist. Take a careful look at the specs on any engine you choose. Note which side the exhaust comes off, which side are the engine controls on (and which direction do they go Push or pull?), and most importantly will the engine fit in your engine compartment?
To figure that out you need to measure everything based off the shaft compared to your A4. I ended up having to raise the stringers 2 1/2 inches and the engine compartment box cover an inch to get it all in and lined up. Most diesels out there are taller than an A4 based on the shaft. But at least on an Ericson that means raising a wooden engine box up a bit, as opposed to my old friend with a Hunter 28 who had to cut a hole in his cockpit sole and raise the sole 2 inches to get a Yanmar in there.
One other important aspect. If you do the work your self, you will know in intimate detail every inch of your piping, plumbing, and wiring in the engine compartment by the time you are done.
Oh yeah, I also had to put in a new raw water through hull. The old one for the A4 was barely 1/4 ID. Kept collapsing hoses and clogging with seaweed from the more powerful raw water pump on the diesel trying to suck through a straw.
Good Luck!
Kevin Wright
E35 Hydro Therapy