The Atomic-4 is a horrible, antiquated piece of equipment. Dozens of sailors are killed each year when these explode, or by carbon monoxide poisoning. The only Atomic-4 boats that have not killed someone, or blown apart, haven't done so merely because the engine is not functioning. The Atomic-4 is known as "The Widow-maker".
The only acceptable auxiliary engine for a sailboat is an inboard Yanmar 2 or 3GM series engine. Accept no substitute.
Seriously, I don't think there are even any statistics available on Atomic-4 explosions or deaths. Countless powerboats operated with gasoline engines and you rarely hear about explosions or deaths on those either, and when you do, it's usually something stupid like burning an open flame during refueling operations.
If the Atomic-4 was such a dangerous POS, then it wouldn't be enjoying the renaisannce that it is right now. Did you know that you can buy NEW engine blocks and cylinder heads from Moyer Marine right now? That's right. The engine is so popular, that these are now being manufactured again.
My Atomic-4 is 40 years old, has never been rebuilt, and it still has factory-spec compression at over 100psi per cylinder. The engine starts every time, and runs like a swiss clock.
Keep your fuel system sealed, your blowers working properly, and your ignition system maintained, and you'll have reduced the risk to a level comparable to diesel engines.
In fact, you'll only have to carry one type of fuel for your inboard engine and your dinghy outboard engine instead of carrying diesel AND gasoline.
You can visit the Moyer Marine forum to learn more, and find a great support community for these engines: Moyer Marine