If you want it to keep time, the pendulum has to swing as fast as the balance would. Typical verge is 4 beats per second although many are faster. Just off the top of my head, a verge needs about 22 degrees to escape and impulse, so you would need an 19 inch pendulum swinging through 22 degrees. It seems doable to me. The weight is easy, as you say, just wrap some high tension fishing line around a smooth barrel the diameter of the fusee using the great wheel of the fusee.