ICE-electric transmission drivetrain parameters
So, would a Petrol&gt;Generator&gt;Motor setup be possible without such
power conversion (directly 3-phase AC from the generator to the motor)
and without electronic speed controllers?

Your idea doesn't make sense. If you are trying to control the speed of an AC motor by controlling the speed of a petrol engine then you might just as well connect the petrol engine shaft directly to the load and save yourself a lot of pointless messing around.
The point about converting to DC and driving an AC motor via a motor speed controller is that you can run the petrol engine at its most power efficient speed and use a control-circuit to produce a massive range of AC drive voltages/frequencies and, get far more dynamic speed range without the need for a gearbox.