Deutz air cooled (well, technically they're oil cooled) engines would have to be one of the most reliable engines around. They're fairly loud, but a very good engine.
On the farm, we had cummins, perkins, international and deutz engines powering centre-pivot irrigators. The Cummins started burning 20 litres of oil per quarter turn of the pivot at 10,000 hours. (It was mistaken for a bushfire once but that's another story) That was replaced with a water-cooled deutz that always, even with a huge radiator, ran hot. The perkins and the inter were good engines, and then there was an air-cooled deutz. That engine was the best of all them - always ran like a charm with zero maintenance, through blsteringly hot and cold, wet days out in the open. The other engines had their issues - they'd leak oil and do idiotic things with their governors etc, but the Deutz never had any of that sort of thing. It just ran.
So, to summarize this rave, if you're looking for a reliable, no-nonsense diesel, you can't go wrong with a Deutz. reasonably smooth running but loud - very loud. You would need to consider whether it would be able to be adapted to on-road use and whether in an enclosed area (vehicle engine bay) you would be able to keep enough air supplied to cool its oil and hence the engine itself. There is a fan that forces air through an oil-cooler but she'd need a fair bit of air. If you find a way around these potential headaches, go for it. You'll have a bulletproof engine powering an ultra-reliable engine.