Replacing an asp control at runtime

Question

I have an existing .NET 1.1 dll that I do not have the source code for. It contains the code-behind for a asp.net page containing various form controls including a third party text editor that I no longer want to use. I want to replace the text editor with a simple asp:Textbox but the existing textbox is baked into the old assembly. How can I override / extend the existing page to use a textbox instead of the old text editor control?