For a generator trailer rather than a pusher you could use just about any motor you like. The RPM of the motor doesn't need to match the cars speed or require a gearbox to achieve the desired ratio.
So why not use a motor with a good power to weight ratio?
If you want to try something a bit different how about a jet turbine electrical generator?
Has a good power to weight ratio.
Scales to any size.
Can be made to run on just about any fuel.
Diesel (including bio-diesel)
Kerosene
Gasoline
Ethanol
Natural Gas
CNG
LPG
Bio-Gas / Methane
So refuelling whilst on a long journey wouldn't be difficult.
Did a quick search and found this one from Bladon Jets but there are probably many more.
12kW output.
Output options, 230v AC , 120v AC or 48V DC.
Actually you probably wouldn't even need a trailer, this could easily be lifted in and out of the car as required. Maybe even mount it where the old exhaust muffler was and have it generate some thrust as well as electricity.
The 134,000 nominal RPM does seem amazing, at least it should be well outside the human hearing range.
