Converting from EditorWindow to runnable window

As you suspected these are not available within the runtime API.

`EditorGUILayout.Popup`

You could use a button and then simulate the popup area using either `GUI.BeginScrollView` or `GUILayout.BeginArea`.

`EditorGUILayout.IntField`

You could simulate this with something like the following (not tested, but you should see the basic idea):

 using UnityEngine;
 using System;
 using System.Text.RegularExpressions;

 public static class YourGUI {

     public static int IntField(int value) {
         // Format integer into string for text input.
         string textInput = GUILayout.TextField(value.ToString());

         // Extract valid characters from input.
         Match m = Regex.Match(textInput, "[+-]?[0-9]+");
         textInput = m.Success ? m.Value : "0";

         // Convert from string to integer.
         return Convert.ToInt32(textInput);
     }

 }

 // Usage:
 yourInt = YourGUI.IntField(yourInt);

`EditorGUILayout.HelpBox`

This is essentially a glorified label. You could achieve this with a custom style (perhaps based upon GUI.skin.box) with three custom icon textures. Here is a pseudo-example (again not tested):

 public enum YourMessageType {
     None,
     Info,
     Warning,
     Error
 }

 private static GUIContent _tempContent = new GUIContent();

 public static void HelpBox(string message, YourMessageType type = YourMessageType.Info) {
     _tempContent.text = message;
     switch (type) {
         default:
         case YourMessageType.None:
             _tempContent.image = null;
             break;
         case YourMessageType.Info:
             _tempContent.image = yourInfoIcon;
             break;
         case YourMessageType.Warning:
             _tempContent.image = yourWarningIcon;
             break;
         case YourMessageType.Error:
             _tempContent.image = yourErrorIcon;
             break;
     }
     GUILayout.Label(_tempContent, yourHelpBoxStyle);
 }

I hope that this will be of help to you :)