I'm pretty sure that Morgan use the Newton range of fuel caps - Newton

I've upgraded my old 4/4 using a their neck and vented locking insert from their classic range.

It may be possible to find a part in their catalogue that is a fairly easy bolt off/bolt on solution.

1972 4/4 4 seater, 1981 MGB GT
1984 Harley Davidson Electra Glide, 1990 Kawasaki ZX10