Switch drivers to loyal members.
Reward your customers for their loyalty by giving them something special.
If you want occasional drivers to become dedicated members you have to make them feel like they will become part of a privileged club. A race license should come with some attractive advantages.
But this is just the beginning.
You can create a unique promotion plan for your drivers based on their racing history at your kart center. This will motivate all your drivers to race more, gather points, money or any other form of loyalty gains.