Here is a patch which gives you all new tracks to race on for Super Mario Kart. Also included are other patches included which tweak the difficulty & game-play. Please refer to the document included. Also be sure to use the Japanese version of the ROM.
Super Mario Kart (Japan) (Rev 0A).sfc
CRC32: C8002453
MD5: F7AFA112D7EC1D532636703E4B02700A
SHA-1: CBB853BF911255C1D8EB27CD34FC7855A0DDA218
SHA-256: C04F517C2675D7A8F3498D958F097443DFAA0E66606C69C59B58A915B3454973