Normally, I would say just remove the broken plug and replace it. But based on how you broke it, and that you are asking here, I'm thinking you don't have a lot of experience and removing a steel bolt from an aluminum housing can often require skill and experience.
You could just replace the cover, but alas Honda no longer offers that as a separate part.
You have a few options.
Remove the water pump cover and take it to a machine shop to have the bolt removed. Order a new flange bolt, sealing washer and cover gasket. Put it all back together and ride on.
Replace the entire water pump, as it comes with everything.
GASKET, WATER PUMP COVER 19226-MN5-000 $5.59
BOLT, FLANGE (6X12) (CT200) 90013-883-000 $1.95
WASHER, SEALING (6.5MM) 90463-ML7-000 $3.48
WATER PUMP 19200-MAM-A60 $210.78
$210.78 vs. $11.02 plus whatever the shop charges you to remove the broken bolt. You make the call.
(Edit) On a lark I found another option, but with the long shipping and such, not worth the hassle in my opinion.
https://www.ebay.com/itm/203308705015
Same with some of the used ones on ebay. Most look pretty rough.