[Arran]

It looks like the brace is loose anyhow, it shouldn't be that hard to remove it, and use it for a pattern to make a new one, provided you have the tools, like a bandsaw, or a jigsaw, otherwise pinning it with a dowel might be worth a try. It looks like the original was pine, or poplar, so you could probably cut a new one out of a piece of 1''x4'', even out of a pallet/skid, a scrap of 5/8-3/4'' plywood would also work.
Regards
Arran