How can I display a scriptable object in a custom editor ?

You can create an editor in OnEnable with Editor.CreateEditor(your scriptable object), and afterwards you can call OnInspectorGUI on that newly created editor.

 using UnityEditor;
 
 public class CustomEditorWindow : EditorWindow
 {
     private Editor m_MyScriptableObjectEditor;
     private MyScriptableObject m_MyScriptableObject;
 
     private void OnEnable()
     {
         // Get a reference to your scriptable object.
         // m_MyScriptableObject = ...
 
         m_MyScriptableObjectEditor = Editor.CreateEditor(m_MyScriptableObject);
     }
 
     private void OnGUI()
     {
         m_MyScriptableObjectEditor.OnInspectorGUI();
     }
 }