Those curves have nearly the same shape for the different voltages, and not a lot of power difference due to being packaged in a similar sized frame. There is only so much power that can be created and dissipated in a certain volume.
They can wind the motor with smaller gauge wire to allow operation with higher voltage, but the current goes lower due to higher resistance. The no-load speed is nearly the same, the peak torque is 29 vs 32 N-m, and the peak output power used to rate each motor is 13 vs 15 kW, not really meaningful differences.
As Major said you really need to get an accurate vehicle load curve (speed vs torque at drive wheel). You can do this with the present motor and battery up to the limits that it will do. Without knowing your requirements you have no idea if you need to double the power of your motor, or increase it 5-fold or even 10x.
Nobody can tell you what size motor you need without seeing some data. The biggest load factors are related to vehicle weight, tire friction and aerodynamics. At the least you need to measure the total vehicle weight, the gear ratio used, the wheel size, and you need to measure the voltage and current while running on a flat level road at several different speeds, like every 5 or 10 mph up to your max speed. You can do some coast-down tests also to get data on rolling friction. Then some math is required--but first you need to get the data...