The proper way is to warm the thing up a bit and remove the filling
is it pitch like. yuo can order replacement pitch from your tool
supplier, you might be able to reuse it if you did not overheat, it
is also used to fill i. e. the handles of table-knives. clean the
leftovers out with Methylated spirit or Aceton (hazardous) than
solder with silver solder, build up if there are any gaps, and than
replate and refill with warm liquid pitch carefully not trappinjg any
air. A hot air gun might be better than an open flame. A difficult
and involved job, but lead solder will not work satisfactory, too
weak and still to hot for the filling. Also you are finishing the
piece off instead of repairing.
good luck and charge enough money forthe work!