Since it is only one flange on the continuous brass flange rail that is
broken, I would suggest using a repair clip of the type sold by Schaff
or American Piano Supply.  One can have the rail duplicated by the
above companies, but I have used many of the repair clips over the
years and have found them satisfactory, sometimes needing just a bit of
filing to fit.

A tiny amount of Titebond glue between the clip and the remaining nub
on the rail will help keep the clip from shifting around.

Bill Maxim