Generator sizing for water pump
The 6.7 multiplier applied to the rated motor current results in 6.7 x 13.5 = 90.5 amps. The current is the most important thing to consider. Since the current at starting lags the voltage by quite a bit, the starting kW will be a lot less than the starting kVA. 90.5 x 230 = 20.8 kVA. You should look for a generator that has a published motor-starting or surge rating published. You should also determine if the generator has a main circuit breaker or other protection that will disconnect the load if the current is too high.

When the motor starts, you can expect the current to pull down the output voltage considerably, perhaps 30 percent. That will be good for the starting situation, because it will reduce the starting current, perhaps by 50%. That voltage reduction will reduce the starting torque. If the load is a centrifugal pump, that should not be a problem. Centrifugal pumps require very little torque to start. If it is a piston pump, you will have a problem.

If the motor is started by a motor contactor with a coil that is connected to the generator output, that contactor may drop out or chatter. You might want to consider a manual motor starting switch.

You should try to borrow or rent a generator that has a surge current or motor starting rating of 50 amps or 10 to 12 kVA. Try that. If it works buy it or one like it.