Welding will not work. That crown is probably aluminum, and possibly (likely) cast, both of which add to the difficulty of welding and post weld finishing such as heat threatment. Not to mention it looks like the broken off piece has threads in it so you will have to tap those and hope they still line up, and then hope it will still fit down over the stanchion.

 If you have any friends at a machine shop you MIGHT be able to get one milled. Even with friends that can do it for cheap it may be worth scraping the fork if you cant find a used one. I guess before I say that I should ask what year it is, I just assumed it was an older one.

 You should try to replace it. It could probably be welded but it's more trouble than replacement.

 Duck tape or hot glue??