Variable Current Draw from motor for torque control
I don't think you realize how much is involved in building such a powerful motor controller. I will explain some basic concepts to hopefully give you and idea of what you are trying to achieve, but I suggest you purchase a speed controller with appropriate ratings. Building a safe and reliable speed controller for 90A is a big deal.

To a first approximation, motor current = motor torque. If the current is flowing from motor into battery (regen) then the torque is acting in the "braking" direction. If the current is flowing from battery into motor, then it is acting to push the vehicle in the desired direction. To produce a regulated braking torque, you will need to produce regulated motor current flowing from motor into battery. This can be done using PWM control in conjunction with a full bridge motor drive circuit. Please note that regen must be limited to avoid charging the battery at too high of a current, and also to avoid overcharging a battery that is already fully charged. Your solution must accommodate these constraints.

In order to control the motor current, you will need some kind of control system which can sense current and adjust PWM duty cycle to produce the desired current. For example it could be a micro controller with features included to facilitate PWM control.

Here is an image of an H bridge which I found online here.



When Q4 is on, the duty cycle of Q1 can be adjusted to apply variable forward voltage to the motor. When this voltage is greater than the back EMF, there will be forward current and the motor will push the vehicle forward. When the applied voltage is less than the back EMF, this will produce regen current and braking.

By adding shunt current sense in the motor path, you can use a microprocessor to control PWM duty cycle to achieve the desired motor current.