I have never seen a stock molding like this.

I could easily make a serviceable molding with three angle cuts on the table saw, but I imagine it would be difficult with a track saw or a router. You would lose the radii, but it probably would not be noticeable.

You could probably make a facsimile by gluing up some stock, but this appears to be easiest on the table saw

Presumably the radius and the slope of the leading edge are designed to direct the water away from the glass. 

My house, with 1950s era windows and a generous roof overhang, does not have a problem with water intrusion.  It depends on the caulk and the slope of the sill to direct the water away from the house.

My parents’ neighbor was one of the first to have storm windows installed in the early 1950s.  The installer caulked really well, sealing all the weep holes.  Five years later all the windows and much of the surrounding framing had to be replaced because of water damage. (Mold was not even on the radar back then, so who knows about that.)

In my opinion, a good seal and provision to have the water drain away from the glass is a reasonable approach, especially on already installed windows.

That was the technology of that era.  It worked well, when properly executed.

Short of replacing the window, that is probably Bugsy is probably going to have to go with.

So the question here, is how does he go about replacing that molding. I think he is going to have to make his own, either by removing stock to come up with the profile, or by stacking and gluing pieces to accomplish the same.