To replace textures you should only create a zip file with textures to replace at the same path as mk64.o2r. Only png, jpg and bmp files are supported.

Example:

You want to change other_textures/logo_mario_kart_64 then you just need to create a zip file with the following structure:

textures_pack.zip
└── other_textures
    └── logo_mario_kart_64.png

Tools To Help

To extract textures from the game you can use convert-texture-o2r. Just place the convert-texture-o2r executable in the same folder as mk64.o2r and drag and drop the mk64.o2r file on it or run it through command line:

convert-texture-o2r mk64.o2r

This will create a textures folder with all the textures extracted from the game.

Future plans

Make name more meaningful and uniform. (example: change texture_69B140). You can suggest name or make a PR to help rename them/organise them.
Support dds files.
move texture in a subfolder.