Here is a
typical generator, if you want to add 10kw continuous power to your traction motor. It weighs 283lbs, converts 18hp to 10kw(13.4hp), for an efficiency of %75 maybe. If the traction motor is %90 efficient then you are looking at
%67 efficiency at getting the output of the ICE to the wheels (ignoring any conditioning of the generator output which would drag it down much farther and transmission from the motor to the wheels).
Even if you add selectable gears to the ICE to drive a wheel it will still be in the %90 efficient range from the output shaft to the wheels and be able to be kept at or near peak ICE efficiency even when not on the hiway.
So in this example, assuming a continuous 12 hp at the wheels is sufficient (can be augmented by battery/motor in both cases where needed), you need a 18hp engine for the serial and a 13.4hp for the parallel versions.