A bit of a long shot but you can probably find a couple of inverter generators, set them up for parallel configuration to get you past 12kW then install NG conversion kits.
Two (2) Genmax 9000 inverter generators gets you around 18,000 peak Watts and 15,200 running Watts (50A hookup built-in)
Two (2) Predator 9500 inverter generators gets you around 19,000 peak Watts and 15,200 running Watts (50A hookup can be built on the parallel kit)
But those wattages are based on gasoline. After NG conversion, you'll likely still get a little over 12,000 running Watts.