Tension-er pulley Bolt

Do you have enough room for a drill?
If so, file the edge of the bolt flat, then use a center punch directly in the center and make a dot. Then start out with a small drill bit, drill all the way through the bolt. You may want to know how far to drill, so purchase the new bolt first. Then step up to the next size drill bit and drill again. Keep doing this until what you have are just basically the thread left, then use an easy out and remove anything left, then run a tap through there to clean up the threads.
If you have no room, see if an angle drill might fit.

I would not know the length of the bolt.
Why did the bolt break?
Did it break while tightening or loosening it?
The reason I ask is maybe an easy out can get the bolt out if it will come loose, but still you have to drill a hole in the bolt, but not all the way through, just enough for the easy out to catch.
If you called the Honda dealer, they can probably sell you the bolt, tell you the part number then Google it to see the dimensions or just ask them to measure it before they ship it out to you.
You would be better off just looking at it to see what would have to come off.
I really do not have any photos that I can look at, unless you want to upload a few photos of that spot, maybe I can tell you what has to come off.