I accidentally clicked on YES on the dialogue screen which asks me if I want to turn off the advertisements where I get +10 seconds at the start of the race. How can I get this back?

The only way to get back the sponsored time bonus is to uninstall, then reinstall the game, which resets all your settings.  If you're using cloud saving, then your game progress should be restored, but the game settings (aka the time bonus) will stay reset to the default.

Make sure to cloud save properly before you uninstall or your progress will be lost.