I couldn't find a direct field for it in the MDL. However, you can use Magos' Model Editor and go to "Windows -> Model Editor". Select all the vertices, then go to "Transformation -> Rotate". As far as I know, it rotates the vertices by recalculating the position based on the angle offset you provide/how many degrees you want to rotate it (calculate offset from center pivot point, then calculate new position (x + offset * Cos(current angle + rotate angle), y + offset * Sin(current angle + rotate angle))). There may be other things involved, I wouldn't know, so I recommend that you use Magos' editor.