Here's a list of currently available new diesels at
Burdin's surplus center.
Note prices -- you pay a lot extra for a "classic".
I don't like them till they get to be water-cooled, myself. That sound of gargling rocks diesels have gets to me. Sad thing is that those tend to be much too large for a home generator, or at least my home -- I'm not running water heaters and so on, and have determined that about 5-6.5 hp is the sweet spot, at least for normal 4 cycle gasoline engines. Less, and there's not enough and it's working too hard, more and it's loafing and wasting gas in friction and low net compression ratio. I suppose you could run any of these more modern engines slower and gear them up a bit to get either 60hz from an AC generator, or to spin the about 4k rpm my old mil surplus fighter plane 24v dc generator likes. That would effectively cut the horses down to about the right number and have the engine running nearer the torque peak, where any engine has the best brake-specific fuel consumption and bonus, it'd probably live longer.
Secondary use -- heat with the hot water you'll produce. Just arrange to dump the waste heat into your basement in winter. I've done this (actually had a generator inside, in a box with a huge fan, even collected heat off the exhaust) and in general, it's enough heat, but I don't reccomend that -- fixing the thing inside the box is a pain, and it's kinda dangerous to have a running engine (and fuel!) indoors -- I made it work in desperation, but I'm no longer that desperate. It did save a ton of money when I needed to do that.
Now, my needs are a lot different than most. My generator never has to handle the peak loads the buildings can produce, ever. It's just charging the big batteries which provide all that, and for me the DC generator works out a lot more efficiently than an AC one that has to then run a battery charger -- fewer conversion losses and no power factor issues (battery chargers off a normal generator draw current and torque only on the sine wave peaks! -- horrible. Even my 20 hp Hobart welder doesn't like it when I turn on the big machines if it's driving them directly. But it's far too much to use as a battery charger -- those things don't like to see huge currents, and like to be charged at C/10 or so max for best life.
Come to think of it though, a big welder might make a nice backup if you use the welding output DC to charge batteries and can adjust that so as not to put in too much current.
I would recommend what I'm doing if someone wants to be serious about the infinite un-interruptible power system though. Trying to run off a generator 24/7 is going to kill it and you both.
And that peak vs average thing means you need a much larger one, which will be very wasteful. At least the power company gets the laws of large numbers working for them, though not perfectly; everyone bakes on Thanksgiving and runs the AC when it's hot, but at least those changes are slow and pretty predictable, and they can spin up other generators when they need them as demand builds. Trying that at home would be more expensive and complex than the battery/solar panels/inverters are, and much less reliable -- very hard to keep in sync.
Orion Birch should be coming along at some point with his adventures with diesel and biodiesel. One of the guys at his commune makes the latter in large quantities, and most of the people there are driving diesels he's built for them. He gets really on the metal, the only electrics are for the lights and glow plugs...tiny alternator and battery to save weight.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.