Any repair will involve at least partially disassembling the front fork top so that you can access the broken off stub area. The only two options you really have is to either buy a new top triple clamp (expensive) or have a small block of hard grade aluminum welded in place of the missing stop- less expensive, but involves a bit of sweat equity on your part.
Obtain a small piece of flat aluminum about as thick as the stop is tall , you have quite a bit of vertical clearance there, and use a hacksaw, a file and a die grinder if you have one to shape it so that it fits in the arc of the steering head . The stops are rather flimsy, not up to a good whack ,as you found, so notch the aluminum to fit around the remaining stub. You can extend it as far as the bosses holding the key and steering lock body wiith clearances left for the welding to same. Remove the steering lock /key body and any plastic bits, sandpaper away all the paint and coating to expose clean,bare aluminum. Have the aluminum surface that contacts the frame stop a bit longer than you need in case after welding you need to file it off a bit to get into square contact with its steel counterpart. When after all this filing, ,measuring and fitting you are satisfied it is a decent fit take it to a welder to get it stuck together.Test fit, final tweaks ,paint, reassemble.Sounds like a project.