Retro mode:

Drive in the town and go to the location of the drag race. Accelerate and follow the markings in the middle of the road to reach a square with a "under construction" sign in the middle. Get some speed and drive straight through the square, which is a jump ramp to reach the clock tower and activate retro mode.

Hint: Treasure Island:

Play in two player mode and choose Chicken Race. You should see a floating rock on your left hand side. Using your fastest vehicle, jump onto it at full speed. You cannot use default Choro Q. If successful, you should reach an island. Carefully go along the beach and you will reach a cave. Inside it, you should see a chest that will talk to you. Note: To read messages, keep both controllers idle.