So good news. I picked up a running D318 engine (thank you for the link Tugger2). I paid $1500. This was a backup generator at a dairy farm for many years. It was just taken out-of-service (to make way for new generator). You push the lever to the low-idle/start setting (which is more of a mid-idle), push the starter, and it fires right up. After it warms a bit, push it over to run, and it holds at 1800 rpm. Pull the lever back all the way, and it shuts off.
I'm very pleased, however - this is not the direct swap I was hoping for. The D318 bellhousing on this generator engine will clearly not bolt up to my CAT-12 transmission. I'm assuming I'll have to swap the bellhousing & flywheel from my old grader engine onto this one (at a minimum). I'd like to run the pony motor also, but there's a turbocharger in the way. The large boss-mount for the electric starter is not present on my grader's bellhousing.
Anyway - here's pictures of the engine.