Repairing zinc die-castings (Zamak, or more commonly pot metal) can be
a major problem.  I have had some success using industrial epoxy and
a brass or aluminum plate bolted on each side.  The only other option
would be to use the old piece as a plug and make a mold.  A home made
gas fired furnace will easily melt aluminum and a new piece could be
cast from that.

Zamak looks much like aluminum and I doubt that most people would
notice the difference.  Aluminum is easily welded if it should need it.

Jim Cook