Your batteries are shot. If you baby your Lead acid and you are lucky you can get up to 5 years of life with a gradual loss of range. If you don't you can get 2 years with a gradual loss of range. Replace the lead with lithium types of cells. If you do this you will most likely have to replace the charger as well. For a lithium battery of the same weight as the current lead pack you will get somewhere between three and four times the range of the new lead battery and it will last you three or four times longer.
A 36 volt battery will be 12 lithium iron phosphate cells which is what I would suggest as a direct replacement.
The speed and torque may be limited by the motor controller. Speed is related to battery voltage. Increasing the pack voltage might be necessary to increase the top speed. You might not be able to increase the voltage unless the motor controller can handle it. You could also change the effective gearing between the motor and the driven wheel to increase the top speed. This will however reduce the torque. The torque an electric motor puts out is proportional to the motor current. The motor controller is probably limiting this to control heat and prevent the motor from burning up or killing the battery prematurely.
In order to get more speed and torque you might have to replace motor, controller, and batteries. Without knowing exactly what all these parts currently are it is impossible to make specific recommendations.
A 36 volt battery will be 12 lithium iron phosphate cells which is what I would suggest as a direct replacement.
The speed and torque may be limited by the motor controller. Speed is related to battery voltage. Increasing the pack voltage might be necessary to increase the top speed. You might not be able to increase the voltage unless the motor controller can handle it. You could also change the effective gearing between the motor and the driven wheel to increase the top speed. This will however reduce the torque. The torque an electric motor puts out is proportional to the motor current. The motor controller is probably limiting this to control heat and prevent the motor from burning up or killing the battery prematurely.
In order to get more speed and torque you might have to replace motor, controller, and batteries. Without knowing exactly what all these parts currently are it is impossible to make specific recommendations.