First stop for seats would probably always be Leightons.
If they don't stock it, they may well have details/designs for one.
That looks somewhat unusual, may need a custom design.
Do you have anything of the original, even the base.
If you can figure out where it attached, and can make a fibreglass seat base shape to suit,
a good upholsterer could make a cushion and cover for it.
Make sure they use hi-density seat foam, not soft stuff that squishes down.
hth.
Pretty looking bike when its in its prime...
