You could try the following to set the type of image and enable it:
// 'widget' is any Widget, SubWidget or Label
// Set the image type. Available options: Car, CarNumber, Helmet, Club
widget.Background.ServerGeneratedBackgroundImage.Type = ServerGeneratedBackgroundImage.ServerGeneratedImageTypes.CarNumber;
// Enable it
widget.Background.ServerGeneratedBackgroundImage.Enabled = true;
However, I am not totally sure it would work, because the images are usually loaded on theme load. I am not sure if they will dynamically load properly if you enable them later, and it could also cause lag spikes. But feel free to try it.