Updating GUIText

You should use the script name instead of MonoBehaviour - unless the first script is called MonoBehaviour.js and is the only one with this name in your project - and assign the result of GetComponent to a variable of the same type (the script type is its name without quotes or extension), then use it as the script reference. Supposing the first script is called LapDisplay.js:

function OnTriggerEnter (myTrigger : Collider) {

 if(myTrigger.gameObject.name == "Start"){
     var scrpt: LapDisplay = gameObject.Find("Lap Text").GetComponent(LapDisplay);
     scrpt.Counter += 1; // that's the way to increment the Counter variable 
 }

} NOTE: You may also notice some hickups when any car passes the Start - searching for some object among a thousand others may be too slow. The fastest way in this case would be to have a GUIText public variable and assign it at Start or at the Inspector instead of look for it every time a vehicle crosses the line:

var lapText: GUIText;

function Start(){ lapText = GameObject.Find("Lap Text"); }

function OnTriggerEnter (myTrigger : Collider) {

 if(myTrigger.gameObject.name == "Start"){
     var scrpt: LapDisplay = lapText.GetComponent(LapDisplay);
     scrpt.Counter += 1; // that's the way to increment the Counter variable 
 }

}

One similar but simple solution is to mark the variable 'counter' as static(which means that it can be accessed from all other scripts) and add to that variable. The only thing you need in order to access the variable is(to mark it as static of course) know the name of the script and the name of the variable you are accessing. So: nameOfScript.theNameOfTheVariable += 1;

 //Call this script Counter
 
 static var counter : int = 0;
 
 function Update (){    
     guiText.text = "Lap: "+Counter;
     
 }
 //And you can call this script what you want
 function OnTriggerEnter (myTrigger : Collider)  {
 
 if(myTrigger.gameObject.name == "Start"){
 //Access the variable by the script name and the variable name
 Counter.counter += 1;
      
      }
 }

NOTE: In your script you are calling "Counter = 0;" That will always result in the variable to be 0 and never change. You should call that in awake so when the game starts the variable will be 0.

One similar but simple solution is to mark the variable 'counter' as static(which means that it can be accessed from all other scripts) and add to that variable. The only thing you need in order to access the variable is(to mark it as static of course) know the name of the script and the name of the variable you are accessing. So: nameOfScript.theNameOfTheVariable += 1;

 //Call this script Counter
 
 static var counter : int = 0;
 
 function Update (){    
     guiText.text = "Lap: "+Counter;
     
 }
 //And you can call this script what you want
 function OnTriggerEnter (myTrigger : Collider)  {
 
 if(myTrigger.gameObject.name == "Start"){
 //Access the variable by the script name and the variable name
 Counter.counter += 1;
      
      }
 }

NOTE: In your script you are calling "Counter = 0;" That will always result in the variable to be 0 and never change. You should call that in awake so when the game starts the variable will be 0.