atomic 4 swap
I did a swap from an atomic 4 to a diesel on my 1970 E-29 about 7 years ago. Due to the availability of rebuildable blocks in my area at the time, I ended up a little oversized - an old Universal M-30, based on a Kubota tractor engine, roughly 24 hp. I was able to 'fit' it in the hole. The major modifications:
Shorten the shaft, build a new engine cover 7" taller than the original, (the cover is now the middle step of the now 2 piece ladder and the aft 'double berth is now a single), change props due to the reduction gear that came with, make a new tank to take the place of the old galvanized one (I fit the a 16 gallon tank into the bilge), biggest project was making a new exhaust system similar to the copper original one.
I love the Universal/Kubota engine - no problems, and when I needed rebuilding parts the local tractor store was more than helpful AND less expensive than the marine engine places. Fuel savings - Atomic 4 roughly 1+ gallons an hour, diesel 3 pints an hour.
If you want more information let me know. -kjs