Custom Editor from a Non-Monobehavior Script

So I am new to a custom editor, but the system I want to write is big for my production. I am essentially working on an item library for the random drops when enemies die, or what a chest/pickup box should be filled with.

I have a simple item script (below) using UnityEngine; using System.Collections;

 public enum ItemType
 {
     Junk,
     Quest,
     Weapon,
     Trap,
     Health,
     AntiVirus,
     Armor
 }
 
 public enum WeaponType
 {
     Ranged,
     Melee,
     Thrown
 }
 
 public enum HandRequirements
 {
     OneHanded,
     TwoHanded
 }
 
 public enum ArmorType
 {
     Head,
     Torso,
     Hands,
     Legs,
     Feet
 }
     
 [System.Serializable]
 public class Item: MonoBehaviour
 {
     //ItemInfo itemInfo;
     public ItemType itemType;
     public string itemName;
     public string itemDescription;
     public int weight;
     public int value;
     public int hpValue {get;set;}
     public int avValue {get;set;}
     public int tier {get;set;}
     public int questID {get;set;}
     public Vector2 attackValue {get;set;}
     public WeaponType weaponType {get;set;}
     public HandRequirements handRequirements {get;set;}
     public Vector2 defenseValue {get;set;}
     public ArmorType armorType {get;set;}
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }
 

and a custom editor, which so far works pretty fine. (below)

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 
 [CanEditMultipleObjects]
 [CustomEditor (typeof(Item)), CanEditMultipleObjects]
 public class ItemEditor : Editor 
 {
     public override void OnInspectorGUI ()
     {
         Item myItem = (Item)target;
         DrawDefaultInspector();
 
         switch (myItem.itemType)
         {
             case ItemType.AntiVirus:
                 myItem.avValue = EditorGUILayout.IntField ("AntiVirus Effectiveness",myItem.avValue);
                 if (myItem.tier > 4)
                 {
                     myItem.tier = 4;
                 }
                 
                 if (myItem.tier < 1)
                 {
                     myItem.tier = 1;
                 }
                 break;
 
         case ItemType.Armor:
             //myItem.armorType = EditorGUILayout.EnumPopup ("Armor Type", myItem.armorType);
             myItem.defenseValue = EditorGUILayout.Vector2Field ("Defense Value", myItem.defenseValue);
             break;
 
         case ItemType.Health:
             myItem.tier = EditorGUILayout.IntField("Class", myItem.tier);
             myItem.hpValue = EditorGUILayout.IntField ("AntiVirus Effectiveness",myItem.hpValue);
             if (myItem.tier > 4)
             {
                 myItem.tier = 4;
             }
             
             if (myItem.tier < 1)
             {
                 myItem.tier = 1;
             }
             break;
 
         case ItemType.Quest:
             myItem.questID = EditorGUILayout.IntField ("Class",myItem.questID);
             break;
 
         case ItemType.Trap:
             myItem.tier = EditorGUILayout.IntField ("Trap Tier",myItem.tier);
             myItem.attackValue = EditorGUILayout.Vector2Field ("Trap Damage", myItem.attackValue);
             break;
 
         case ItemType.Weapon:
             myItem.attackValue = EditorGUILayout.Vector2Field ("Attack Value",myItem.attackValue);
             break;
         }
     }
 }
 

The problem is, I want to turn the item script into a non-monobehavior class and have it accessed as an array of "Item" from another script. When i do that the editor script throws an error that I do not know how to fix. (below)

Cannot convert type UnityEngine.Object' to Item'

If anyone knows how to fix my problem and/or a better way to go about writing this please let me know. I appreciate your time!