How can i avoid breaking LSP in this example? C#

I have a base class called Message like this:

public abstract class Message 
{
    protected int m_id;
    protected bool m_localized;
    protected string m_metaData;

    public int GetID() { return m_id; }
    public bool GetLocalized() { return m_localized; }
    public string GetMetadata() { return m_metaData; }
}

Then, i have two more classes that inherit from Message for example:

public class ClassicMessage : Message
{
     private string m_title;
     private string m_content;

     public void SetTitle(string title) { m_title = title; }
     public void SetContent(string content) { m_content = content; }
     public string GetTitle() { return m_title; }
     public string GetContent() { return m_content; }
}

public class MessageWithCustomContent : Message
{
     private List<CustomContent> m_content;

     public MessageWithCustomContent() 
     {
          m_content = new List<CustomContent>();
     }

     public List<CustomContent> GetContent()
     {
          return m_content;
     }

     public CustomContent GetCustomContent(int id) 
     {
          return m_content.find(x => x.ID.Equals(id));
     }
}

public class CustomContent
{
     private int m_id;
     public int ID { get; set { m_id = value; } }
     private string m_body;
     public string Body { get { return m_body; } set { m_body = value; }
     private Image m_image; 
     public Image Image { get { return m_image; } set { m_image = value; } }
}

In a case like this, how can i unify the program interface if the derived classes has similar methods but these methods have different return types? (even when the methods try to do the same)

I know that with the example i'm breaking the Liskov Substitution Principle, what's the best approach to get around with that?

Thanks for your help!