When you start racing, you move automatically, as shown by the clouds. The space key just gets you moving faster. Your adversary (who's appearance changes - spam the green flag to watch this in action) will be ahead, then behind, then ahead, then behind, then ahead, then behind, then ahead, then behind... To avoid the exact same outcome, I used a random block to make the race between 15 & 30 seconds. Lastly, use the # keys (1-6) to change your appearance.
All notes are above. Everything by me. I've shared a 2P Version! https://scratch.mit.edu/projects/265943707/