How to add touchscreen controls to this script?

Hi, new to Unity. Need help with university project due in next week.

The game is a plane flying game and I want to add two onscreen touchscreen buttons for acceleration and braking (not too bothered about braking, more bothered about acceleration).

Any help would be greatly appreciated. Thanks

p.s I didn't write this script

 var gameover=0;// Game Over || Turn on and off the airplane code. Becomes interesting at a later point. Game over ...
 static var airplaneangley: float=0.0;// This goes to the blob shadow. 
 
 // Rotaton and position of our airplane
 var rotationx=0;
 var rotationy=0;
 var rotationz=0;
 var positionx: float=0.0;
 var positiony: float=0.0;
 var positionz: float=0.0;
 
 var speed:float =0.0;//speed variable is the speed
 var uplift:float =0.0;
 
 function Update () {
 
 //Shadowstuff-----------------------------------------------------------------------------------------------------------------------------------------
 
         //shadow to the same angle than the airplane
         airplaneangley= transform.eulerAngles.y; 
 
 //Maincode flying ----------------------------------------------------------------------------------------------------------------------------------
 
     //Code is active when varwhatmoves is 1
     if(gameover==0){
 
     // Rotations of the airplane
         if (speed>595)transform.Rotate(Input.GetAxis("Vertical")*Time.deltaTime*100,0,0); //Hoch Runter, limitiert auf eine Minimalgeschwindigkeit || Up Down, limited to a minimum speed
         transform.Rotate(0,Input.GetAxis("Horizontal")*Time.deltaTime*100,0,Space.World); //Rechts Links || Left Right
         if (groundtrigger.triggered==0) transform.Rotate(0,0,Input.GetAxis("Horizontal")*Time.deltaTime*50*-1); //Seitenneigung. Mal Minus 1 um in die richtige Richtung zu drehen || Tilt multiplied with minus 1 to go into the right direction
         
         //limit tilt so that airplane doesn`t fly a roll while flying a curve
         if ((Input.GetAxis ("Horizontal")<0)&&(rotationz >45)&&(rotationz <90)) transform.Rotate(0,0,Time.deltaTime*-50);//linksrum || to the left
         if ((Input.GetAxis ("Horizontal")>0)&&(rotationz <315)&&(rotationz >270)) transform.Rotate(0,0,Time.deltaTime*50);//rechtsrum ||to the right
 
     //Speed
         transform.Translate(0,0,speed/20*Time.deltaTime);
     
     
     //Turn variables to rotation and position of the object
         rotationx=transform.eulerAngles.x; 
         rotationy=transform.eulerAngles.y; 
         rotationz=transform.eulerAngles.z; 
         positionx=transform.position.x;
         positiony=transform.position.y;
         positionz=transform.position.z;
 
     
     //Rotate back in z axis , limited by no horizontal button pressed
         if ((rotationz >0) && (rotationz < 90)&&(!Input.GetButton ("Horizontal"))) transform.Rotate(0,0,Time.deltaTime*-50);
         if ((rotationz >0) && (rotationz > 270)&&(!Input.GetButton ("Horizontal"))) transform.Rotate(0,0,Time.deltaTime*50);
         if ((rotationz >180) && (rotationz < 270)&&(!Input.GetButton ("Horizontal"))) transform.Rotate(0,0,Time.deltaTime*-50);
         if ((rotationz <180) && (rotationz > 90)&&(!Input.GetButton ("Horizontal"))) transform.Rotate(0,0,Time.deltaTime*50);
         
         //Rotate back X axis
         
                 if ((rotationx >0) && (rotationx < 180)&&(!Input.GetButton ("Vertical"))) transform.Rotate(Time.deltaTime*-50,0,0);
                 if ((rotationx >0) && (rotationx > 180)&&(!Input.GetButton ("Vertical"))) transform.Rotate(Time.deltaTime*50,0,0);
         
         
         //Speed driving and flying ------------------------------------------------------------------------------------------
         //
         //We need a minimum speed limit in the air. We limit again with the groundtrigger.triggered variable
     
         // Input Accellerate and deccellerate at ground
         if ((groundtrigger.triggered==1)&&(Input.GetButton("Fire1"))&&(speed<800)) speed+=Time.deltaTime*240;
         if ((groundtrigger.triggered==1)&&(Input.GetButton("Fire2"))&&(speed>0)) speed-=Time.deltaTime*240;
         
                 //  Input Accellerate and deccellerate in the air
         if ((groundtrigger.triggered==0)&&(Input.GetButton("Fire1"))&&(speed<800)) speed+=Time.deltaTime*240;
         if ((groundtrigger.triggered==0)&&(Input.GetButton("Fire2"))&&(speed>600)) speed-=Time.deltaTime*240;
         
         //Auftrieb  -------------------------------------------------------------------------------------------------------------------------------------------------------
         
         //When we don`t accellerate or deccellerate we want to go to a neutral speed in the air. With this speed it has to stay at a neutral height. Above this value the airplane has to climb, with a lower speed it has to  sink. That way we are able to takeoff and land then.
         
             // Neutral speed at 700
             //This code resets the speed to 700 when there is no acceleration or deccelleration. Maximum 800, minimum 600
         if((Input.GetButton("Fire1")==false)&&(Input.GetButton("Fire2")==false)&&(speed>595)&&(speed<700)) speed+=Time.deltaTime*240;
         if((Input.GetButton("Fire1")==false)&&(Input.GetButton("Fire2")==false)&&(speed>595)&&(speed>700)) speed-=Time.deltaTime*240;
     
         
         //uplift
         transform.Translate(0,uplift*Time.deltaTime/10.0,0);
                 
         //Calculate uplift
         uplift = -700+speed;
 //We don`t want downlift. So when the uplift value lower zero we set it to 0
         if ((groundtrigger.triggered==1)&&(uplift < 0)) uplift=0; 
     }
 }