You can edit this text via a Resource Editor such as ResEdit.
Inside the file "Scenario" that resides inside each scenarios folder is a resource of Type "TEXT" Edit this text as you wish and give it an ID of -200 through -300. Open the "Scenario" file with ResEdit and you will see a resource of type "TEXT" that you can edit.
In the "Map Editor" place the ID in the "Scrolling Text ID" field and this text will be displayed as a scrolling message instead of a normal map.