Sega Master System / Mark III / Game Gear
When the game enters its "scroll to the player" mode (at the start of a race, when you respawn or go through a pool table), it will now get stuck in there. The D-pad controls the target position. Press to resume the game.
Levels are stored in ROM at $3a684 for Super League mode, in the order 1-1, 1-2, 1-3, 1-4, 2-1, ... followed at $3a694 with the 19 stages of Challenge mode. 2-player mode lets you choose from the 19 bytes from $3a684, since the 20th is a duplicate level.
There are three unused levels: two "basement" stages with sprites but no stage graphics or data, and one sports cars stage "Turbo Turns" which seems complete.