Any advice on how to repair the drain cracking?

What is the old material? You must achieve a pressure-tight bond to it, so
it is important to know what it is.

If it is fiberglass/epoxy, I would lightly lube the threaded plug, clean
the crack, insert the plug, and fill the crack with epoxy, working it in
with a toothpick. Remove and reinsert the plug with Teflon tape. Do not
apply to much torque.

Another candidate is 3M 5200 sealant (boat store or boat aisle at Wal-Mart)
in the quick-cure grade, similarly applied.

If there's enough meat around the fitting, you could drill and tap a larger
hole.