Adding stamina to existing script

I found a script that adds sprinting, crouching, etc to CharacterMotor.js. Now i'm trying to add a stamina bar and am having difficulty.

sprint.js

 var walkSpeed: float = 8;
 var crchSpeed: float = 3;
 var runSpeed: float = 16;
 var stamina : int = 100;
 
 private var chMotor: CharacterMotor;
 private var ch: CharacterController;
 private var tr: Transform;
 private var height: float;
 
 function Start() {
     chMotor = GetComponent(CharacterMotor);
     tr = transform;
     ch = GetComponent(CharacterController);
     height = ch.height;
 }
 
 function Update() {
     var h = height;
     var speed = walkSpeed;
     if (ch.isGrounded && Input.GetKey("left shift") || Input.GetKey("right shift")) {
         if (stamina != 0) {
             speed = runSpeed;
             stamina = stamina - 1;
         }
     }
     if (Input.GetKey("left ctrl") || Input.GetKey("right ctrl")) {
         h = 0.5 * height;
         speed = crchSpeed;
     }
     chMotor.movement.maxForwardSpeed = speed;
     var lastHeight = ch.height;
     ch.height = Mathf.Lerp(ch.height, h, 5*Time.deltaTime);
     tr.position.y += (ch.height-lastHeight)/2;
     
     if (stamina != 100) {
             stamina = stamina + 0.5;
         }
 }
 
 function OnGUI () {
     GUI.Box(Rect(120,Screen.height - 50, 80, 40), "");
     GUI.Label(Rect(130,Screen.height - 30, 180, 125), stamina + " / " + "100");
     GUI.Label(Rect(130,Screen.height - 50, 180, 125), "Stamina");
 }

The GUI and depletion of stamina, works. Recharging stamina, does not work. I would also like to fix stamina decreasing when shift is pressed but the player is not moving.