I would knock both the key remnants out and check the keyways on the crank snout. From the pics, it looks like the keys just sheared in half. You might just get by with new keys and a balancer, if its keyway was distorted.

The keyways on my 406 were "crunched" on one side by the keys. I was able to put new keys in and fill the gap with JB Weld. After a day, I gingerly replaced the balancer and bolted it to spec. It's held for two years.