Since new MGA Coupe headliner bows are not available you might want to make your own. Here is a preliminary drawing (subject to change). Dimensions are in inches. Click for a larger PDF image. If you can print it to scale you can use it as a template for forming the new parts.

Drawing in process.
At first glance, these may have the same cross section as
the rear window braces for the Vandenplas aluminum hardtop.

On 6/27/2018, Niklas Berge wrote:
"I made the bows for the roof lining according to the sketches and drawings we made and got them near correct, only had to move the holes by <1 cm. I think you better do them and drill the holes when the bows sneak along the roof".

"I used the lining from Moss, and to get a proper fit I had the arcs following (close) the roof line (except for the last ~4”). I think I had to shorten the front one by moving the holes ~10 mm on one side, still having the arc following the roof line. They were made by curtain rods which was first flattened a bit in the vice, then used wood formers to get it close to the original".