Common round 20lb balance weight, including bracket and lever. Spare weights are SC042/1 (a 7mm scale product).
The weights should be sufficient to restore the arm to the horizontal. If not, try using SC142 instead, or configure the operating mechanism to move the arm in both directions. (Note that on the prototype, the function of the balance lever is to keep the wire from the signal box taut. If anything breaks, the difference in weight between the arm and the spectacle casting restores the arm to danger/caution.)