Here's an easy way to get the top KO's (unlock Darth Maul):
First, win the tournament with Sebulba which unlocks Sebulba Episode1 (his stats will better than any other normal racer). Go to the single race option at the start menu and select him as your racer. You're alot faster than anyone else so go to the front of the pack and start knockin em down. Once you've got the top KO on all levels, go back to the start menu select tournament and Sebulba as the racer. After you finish your first race a screen will pop up saying you've unlocked Darth Maul.