I went to Advance Auto Parts with my original headlight in hand, and bought a 5-3/4" sealed beam headlight. Sylvania H5006. The guy behind the counter shrugged his shoulders, and said, "Yeah, they're the same." $14. Then, I went up the street to AutoZone. I asked if there is any difference between motorcycle headlights and car headlights. The guy there said, "No." I showed him my old light, and he matched it up with a Sylvania H5006. I bought it. $16. Doesn't hurt to have a spare.
Then, I went by my local motorcycle repair shop. I had noticed, of course, that the original light has "MOTORCYCLE" molded into the lens. I figure there's a reason for that. So, I asked Kenny if there's a difference in the construction of car headlights and motorcycle headlights. I think the motorcycle headlights must be a little more robust to withstand vibration. He agreed. Unfortunately, he doesn't stock 5-3/4" motorcycle headlights. He told me that he could order a conversion kit for me so I could use those H4 replacement bulbs, but he seemed reluctant to give me an idea of how much that might cost.
When I installed one of the new headlights, I noticed that there are three or four nubs on the back of the perimeter of the lens. Presumably, these are to ensure proper orientation. The original light has only one alignment nub, and the housing has only one indentation to accept that nub.
I really didn't think that getting the right bulb would require such a process. I can't even find the right light on the Internet. So, I've got one of the new H5006's installed. It's just not properly seated in the housing, and so it's probably pointed a couple degrees off center.
Another thing that has surprised me is the difficulty in finding an LED-filled headlight on the Internet. I mean, it is 2014. I would think a 5-3/4" low-amp LED sealed beam headlight, with the same little nub on the back would be easily found.