I'm not exactly sure where to post this, since I used a Wii to test out my hacks and since the Wii has Gamecube support...mind as well post it here.
This was done by modifying offset 0x18 in the course data BOL, and this value stores how many laps there are in the race. 1, 2, 3, 4, 5, 6 and 7...