So, the XR650L fender brace worked, overall. [I'll upload pictures in a second reply]
The shape and the bolt pattern are a match and this worked fine. The result is very stiff.
To mark the drill points, I put the brace in the installed configuration above the fender and used a marker through the brace holes while clamping the brace against the fender.
I used a 5/32" drill bit through the fender, after drilling a smaller guide hole.
What did not work: the included replacement bolts to attach the fender with brace back to the fork brace were not a match. So, I used the existing bolts, removed the washers, and gave them some extra Loctite to secure them until I find a good bolt match with more length to help secure them better.
Another concern is that the bottom of the headlamp now is very close to the fender. I think this is fine given the stiffness, but it did mean removing the headlamp to install the brace bolts.