Moving a GameObject with a script from another GameObject (Rigidbody)

Hi guys, I'm trying to have a GameObject move using it's rigidbody from a script. I seem to be unable to do it from another GameObject. This script is attached to "_GameController":

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class Boundary{
     public float xMin, xMax, yMin, yMax;
 }
 
 public class PlayerMovement : MonoBehaviour {
 
     public Boundary boundary;
     public float tilt;
     public float speed;
     public Rigidbody player;
 
     void Start(){
         player = GameObject.Find("ship").GetComponent<Rigidbody>();
         boundary.xMax = 8.5f;
         boundary.xMin = -8.5f;
         boundary.yMax = 4.5f;
         boundary.yMin = -4.5f;
     }
 
     void FixedUpdate(){
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
         Vector3 movement = new Vector3(moveHorizontal, moveVertical, 0.0f);
         player.velocity = movement * speed;
         player.position = new Vector3
             (Mathf.Clamp(rigidbody.position.x, boundary.xMin, boundary.xMax), Mathf.Clamp(rigidbody.position.y, boundary.yMin, boundary.yMax), 0);
         player.rotation = Quaternion.Euler (0.0f, 0.0f, rigidbody.velocity.x * -tilt);
     }
 }

This script is meant to make "player" move. I know it's setting the variable right but it's telling me that "_GameController" has no rigidbody even though I'm not even trying to access it. Any ideas?

Now I know you will tell me to just put it on the Object i desire to move but the thing is, the object is a ship and the player can buy many ships with different speeds and everything so it would require a lot more work.