Do you have any idea what was causing the problem to begin with? It seems odd that the balancer would become loose to start with. Could the pulley have been bent which was putting odd pressures on the balancer? Any indication of other damage? It may be worth experimenting with a new (used) balancer assembly before giving up. My Mercedes manual states that on the 617 engine the vibration damper can be replaced without balancing and the balance plate can be static balanced. How badly damaged are the grooves for the dowel pins?