Teleport your worms at the start - Start At Start rule. Rope around the map in the direction you wish as long and fast as you can and count the number of laps finished (having a careful spectator counting laps could be nice). A lap is finished if you passed the Start line.
First players reaching 20 laps win and proceed to the next round. The game ends when everyone played the same amount of turns.
If you plop your worm, the current lap doesn't count. If you plop all your worms, you lose.
If a draw happens (i.e. if two players needed the same amount of turns to finish their 20 laps, but only one can proceed), replay a game for the players affected by it: the winner is the first one who accomplished 10 laps. If a draw happens again, then it's sudden death: keep playing until someone gets slower than the other, giving both players the same amount of turns.