Mechanical to electric to mechanical.... Power loss to expect
You seem to be describing a system with an engine that can provide 2 kW at the proper speed to drive a generator. The output of the generator would go to four power conversion speed controllers that would provide power to four wheel motors. For estimating purposes, the generator might have a 90% efficiency. The speed control units might have 95% efficiency. The wheel motors might have 85% efficiency. The system efficiency would then be 0.9 X 0.95 X o.85 = 0.73. The full-speed efficiency would then be about 73%. You could drive the wheel motors at about 3/4 of their rated power.

The speed of the engine can not really control the speed. It would be best to operate that at the most efficient operating speed, presumably close to full speed. The wheel motor controllers must control the speed. The power actually used to drive the cart will be reflected in the torque load on the engine.