Here is a full hack for Super Mario Bros. 2 which changes all the levels and some of the graphics in the game. Also some degree of difficulty has been added such as normal encounters with Mouser and harder levels :).
Super Mario Bros. 2 (USA) (Rev A).nes - NOINTRO
CRC32: E0CA425C
MD5: 25B7E8471AE58316593E58205FA16803
SHA-1: 67F68CABC5237A5892AFF746F5B784C62FCCDFAC
SHA-256: 6CA47E9DA206914730895E45FEF4F7393E59772C1C80E9B9BEFC1A01D7ECF724