If there’s enough of the broken bolt exposed to grab with channel locks, or to file a slot in for a large screwdriver, you might be able to remove it and replace the drain bolt.
Best course of action may be to simply replace the water pump cover, drain bolt and washer.
#2
COVER, WATER PUMP:
19221-MZ0-305
#8
BOLT, FLANGE (6X12) (CT200)
90013-883-000
#10
WASHER, SEALING (6.5MM)
90463-ML7-000
