Here is a patch which converts the mapper to MB5.
Super Mario Land 2 - 6 Golden Coins.gb
CRC32: 635A9112
MD5: 4BD6E929EC716A5C7FE7DC684860D551
SHA-1: D11D94FA3C36B9F72E925070B66BB4F16D31001E
SHA-256: 42045A1642F77B568534AB85B9DB93C8A33D2B95C38AE84C987F497E8D52F97F