That was probably put on by the dealer, so not technically a Sea Star Mustang light.
Find someone w a 3d printer and ya might be able to print a uv stable, durable lense that would work. Scour Fiberglassics library for a pix of 1 that looks similar & check their hardware listing, might get lucky & find a pix there too. And design your own lense in CAD
OR, find a suitable vintage light and swap it out for a complete unit, as stated, not a factory light so other Mustangs sold by different dealers likely used a different light.
Place a want ad @ Fiberglassics classifieds
Spend 2yrs scouring ebay vintage boat hardware/lighting listing, BTDT....