The top of the steering stem is 15/16 inches. The stock Ruckus handlebar is not adjustable. It is bolted through the stem so the angle can not be changed without replacing the handlebar entirely.
To mount a different handlebar, a clamp is required. Some mountain bicycles have 1 inch clamps according to this thread on Ruckus Central. Here is a picture of a Ruckus with a mountain bike stem.