[oil pan 4]

I would not recomend trying something like this with a budget generator.
They are very noisy, make lots of vibrations, suck down fuel as if there were a hole in the fuel tank and they tend to have short lives before encountering their first problem (100 to 500 hours).

A 4 pole generator would have to be used (1800 RPM) to get the engine to run fairly quiet and to get fuel consumption down to acceptable levels. These 4 pole generators tend to have very long life due to much lower vibration I think.

For a gas engine you would need something like a 20hp kolher V-twin, that way you can get the 12hp or so you need to drive the generator down at lower speed (1800 RPM).
For a diesel a smallish >1L 3cyl kubota diesel would work. Add a turbocharger and get an additional 10% more fuel economy.

A 3phase generator would further reduce engine load. I was reading that a 3phase generator takes about 12% less engine torque to turn than a single phase putting out the same load.
Plus 3phase makes much better DC power than single phase with out a lot of other stuff.