Here is a patch which gives you all new tracks to race on for Super Mario Kart. 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