The effect of high frequency on an induction motor.

Above the base motor design speed, you are running in what is called "constant HP mode" because you can no longer maintain the designed V/Hz ratio. So since HP is a function of torque and speed, if the speed increases and the voltage cannot, you start losing torque and the HP stays the same. Your 20HP 50Hz motor is still a 20HP motor at 70Hz, but will provide only 71% of it's rated torque (50/70) at that speed. The end effect of that depends totally on the nature of the connected load.

If your load is mainly speed dependent, such as a polisher or grinder, that may not make any difference.

If your load is a constant torque load, such as a conveyor, you will not have enough torque unless the motor was seriously over sized.

If your load is a centrifugal pump or fan, load power varies by the cube of the speed (quadratic load) so at 70Hz you are at 140% speed, so your power requirement went up by 1.4² to almost 40HP. Your motor will overload.