That was kind of the point behind using the ICE from the donor car and the bigger motor from the forklift
Seriously, roughly how many KWs does it take to maintain 65 in a reasonably slippery car? What would that equate to in HP? I realize that we have to factor in efficiency losses but that is a starting point.
My only real concerns about the original ICE is that A) it may be heavier than needed.....but not sure if a smaller industrial diesel would really be any lighter. and B) if the engine's most efficient output range can't be matched to the generator's best speed whether through a chain drive or whatever. The weight can be dealt with possibly by looking for the smallest automotive diesel I can find, and the speed rating can at worst case be solved by using a bigger generator that can efficiently use all of the ICE's power at best RPM....and then shut it down when it is not needed.
Considering that unless you're a traveling salesperson most cars spend a fraction of their time on long road trips, I can accept a small trade-off in highway efficiency if it makes the car practical to use in daily driving as a primary / only car.