1 kW is likely the constant power rating of the motor limited by overheating, not the peak power. So peak current would be higher than 21A, likely limited by the peak current of the motor controller at low motor rpm and by the low pack voltage at higher rpm due to back emf of the motor. The latter will be worsened by sagging of the pack voltage at higher currents and will likely limit the speed on hills.
You will need to know the current the motor now draws going up a given hill, the rated peak current of the controller, and how that compares to the peak current your 100Ah batteries can supply, in order to see if a larger motor will provide any benefit. It could be that the lower power motor doesn't have sufficient torque, but I think it more likely that the controller or batteries are the limitation.
If you have no current meter, but have an Ah meter you could get a rough estimate of current by driving up a hill at constant speed (the speed you approach at from the bottom, so no complication due to acceleration) and recording Ah reading at the top and bottom of the hill, taking the difference and dividing by the time taken to drive between the two points. You will need a long, steep hill to consume enough Ah to get sufficient sensitivity in the measurement, less so if the meter reads to the nearest tenth Ah. The vehicle may slow as you climb the hill, but that's ok, the discharge current will likely remain about constant at the peak the controller or batteries can supply.
The Peukert Effect mentioned decreases battery capacity at higher discharge currents. So for example the Trojan T-125 has 240Ah capacity at the 20 hour rate, but has only 195Ah at the 5 hour rate (4x the discharge current).
The larger motor will not cause a significant change in energy consumption if the vehicle is operated the same as with the current motor.