The cores usually break because the steel center post rusts and swells up. As mentioned above, just glue them back together and lightly sand the exterior so that the slug doesn't bind in the form. Then realign.
Once upon a time I was working on an R-388 and found two broken slugs. "Take the bottom panel off, retrieve the broken pieces and glue them" I thought. Found the broken pieces, but...another and another, way more than the pieces I needed! Checked
every other slug, none broken. Evidently some depot or shop had repaired broken slugs before me, and had just installed new ones and left the broken pieces lying inside.
If you can't find the broken off parts, at least a few years ago Fair Radio said they had pull-out slug and rod assemblies available.
