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Small and cheap engines are built to be just that - small and cheap. I can't comment as much about efficiency but I can say something about reliability.
Some rules of thumb based upon when I was researching off grid power and generators... a typical "small gas" engine will last 700-1000hrs before theyre done. It's possible that some maker (maybe Honda if anyone) makes a higher quality longer lasting product, but your typical lawnmower sees such intermittent use (what, 2 hours every week?) that they last for years and give the mistaken impression of being reliable. Try one in commercial service sometime. A 3600rpm gas genset will wear out the engine within just a couple weeks if it's on 24/7.
The high speed 3600rpm diesels are better, but not hugely better. They are made to be cheap again but have to be built heavier due to pressures and such. However because they are more expensive, no more powerful, and usually made by GOOD manufacturers (Yanmar) they will outlast gas engines by maybe several times maybe more. I have the least research information in this category though.
The "medium speed" 1800rpm diesels is where many of your industrial diesels come into play - Hatz, Perkins, etc. This is what you find used in say farm utility tractors, those weighing 5000lbs and meant for heavy duty constant commercial or farm and ranch use. This is not your Sears Gardenmaster (which with its 3600rpm gas engine will die in 700-1000hrs) or whatever, these will still be running after hard use. They cost waaaay more than gas engines of similar power, weigh quite a bit more (they have to make the power at half the rpm) and are made for truly constant use. 10,000hrs of use is probably not unreasonable to expect by what I understand. These actually will run your genset head 24/7 all year if you need it to.
If I wanted to build some small trike or runabout I would probably look at motorcycle gas engines that have a reputation for long life. Yet even those seem lacking compared to many cars.. There are motorcycles that are almost worn out at 10,000 miles and those that will go 100,000 miles. Yet nearly any car will go 100k miles.
I've seen people put single cylinder Hatz industrial diesels (typically 3600rpm types) or Yanmars into motorcycles, I would imagine they would return the longest life and the best economy possible. Were I to build a faired scooter or similar thats what i'd want to put in personally. (and ideally with some kind of hydraulic hybrid using an accumulator since acceleration is where they are a bit deficient)
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