Is it possible to use a MAX232 to convert 0 to 5V PWM to 0 to 12V PWM to input to a motor driver requiring 0 to 12V PWM logic?
It is highly unlikely that it will be worth it to convert a MAX232 circuit to drive this PWM.
MAX232 in general uses the supplied 5V to make +/- 7 volts with a charge pump, it might be possible to make it drive 0V and 12V, but it would require an external 12V supply, and while being within absolute maximum ratings, this kind of configuration is not shown in datasheets or application notes.
Perhaps it would be better to use a FET gate driver for this, they are designed to take in logic level signals and level convert them into higher voltages such as 12V.