Kirk,
Some drivers/tuners intentionally cut their seat up there to increase the chassis performance. See how it drives and go from there. You may be limiting your flex by repairing it and hurting the overall performance.
You will need to test and see. My suggestion: If your kart handles well, and you are comfortable with it, leave it alone. Keep an eye on it to make sure it does not get larger. We would hate to see you fall out of your machine during the heat of the battle.