Good Luck on the Repower -
I recently did a repower on my E29, I believe it's essentially a stretched E27. I put in a BETA Marine BD722 Diesel and couldn't be happier. This is a Kubota Based 3 cyliner Diesel, very smooth, fuel effiecient, 100 lb's lighter than the A4, 1/3 gallon per hour, and higher top end than the A4 provided.
I'm pretty sure you would be more than adequately served by the BD522, which is a 2 cylinder 13 hp version of my engine. I suspect you're going to pay around 5k for the engine.
It is a potential do it yourself job, but there is some heavy work involved. Here's my thoughts in bullet points:
1. Get the old A4 out, the coupling may be frozen which is a bear to undo, Don Casey's book has some good tips on how to get it free.
2. Get the engine out, the A4 is a good 300 pounds. You can rig a topping lift to your boom, rig a block and tackle where the topping lift is connected and lift it out. Or, pay for a lift at the boat yard crane. Pay for the lift is safer.
3. You're going to need to raise your engine bed to get the shaft angle correct. The two shaft transmission set's the diesel up higher. In my boat, we took white oak stringers and lagged them to the existing stringers which seemed ok. We throughly soaked with Smith's penetrating epoxy, put a heavy epoxy based paint on them, and bedded in 3m 5200, then bolted. They are not going anywhere.
4. Mount the new engine mounts to the stringers. Lots of little movements and adjustments..., this is the most important part. The replacement engine is light enough that you can slide it around by hand.
5. You'll probably need a new shaft coupling to fit the bolt pattern. I upgraded to a 1 inch shaft, it's a tight squeeze throught the shaft alley, but it fit in my strut and was well worth it.
6. You'll need to about double the pitch in your shaft, the diesel has a 2/1 reduction gear.
7. I recommend new control's, Morse has a very nice single lever unit that fits in smoothly.
8. Get the best fuel filter you can get, I recommend racor turbine series.
9. You'll need to rig a fuel return line. Actually, you need to replace all fuel lines. Very important to get your fittings real tight and solid, an air leak will kill the Diesel like you shot it.
10. The engine comes prewired. Install the panel and plug and play.
Hope this helps.