Overvolting a DC motor while keeping power constant
The since the speed is directly proportional to voltage, the motor will attempt to run at 133% of rated speed. If you are driving something like a fan for centrifugal pump, the torque load on the motor would increase to 1.33 X 1.33 = 1.77 pr 177% of rated load. The increased load will prevent the motor from going that fast, but it is still likely to be overloaded and overheat rather quickly. The driven equipment might not fare too well either. Look for or design a speed controller that will limit he voltage to 36 volts.

It is also possible that the commutator will have arcing among the segments with increased voltage. The winding insulation will probably not have a problem.

Re comment

With traction applications, the load torque will increase with speed to the extent that the vehicle is subject to aerodynamic drag. Increasing the voltage does not increase the torque capability of the motor. Therefore, any load that requires more torque to operate at a higher speed has the potential of overload the motor if it is operated above rated speed. With a traction application, I would think that you would need a speed controller. I see that you are intending to use a chopper. That should be configured to adjust and limit the speed and limit the current. Since it controls the voltage by controlling the duty cycle, but the peak voltage would still be 48 V. There may be no problem with the commutator, but you should look to see if there is any problem.