I couldn't see a cloud tileset for SMB1 in any GFX pack so I decided to make that. It is my first submitted tileset. For decorating the cloud you can put some cloud faces which replaces background-106 (SMB3 Yellow Foreground Bridge), otherwise it will look so bland.
http://www.mediafire.com/download/na404 ... ileset.zip
I know some people makes level for contest. This tileset may help you to give you inspiration.