Laser or a PUK/Orion welder. Make sure you clear out the wax for a
few MM either side of the joint, so the heat doesn’t cause it to
melt into the joint area while you’re welding.
And take your time. The extra time you spend letting the joint cool
before you take the next few shots will be more than saved by not
having to deal with melted wax in the joint if you get impatient.
Electroformed stuff tends to be pretty thin, so it may never be all
that strong. (it wasn’t the first time either, which is why it
broke.) You might try stuffing a toothpick into the core of it, into
the wax on either side of the break, sort of like an arm-bone. Won’t
make it bulletproof, but it can’t hurt.