I'd think a pattern could be made from an existing top on another car, but would be prohibitively expensive to go that route. Probably your best bet would be to find another owner who has an existing top in need of replacing, and go together on a pair of tops made from that one. Even then, each one would have to be custom fitted separately. Having the original would be key to the restoration.
I was lucky with my '38, the top had been made by a local club member who was the best in his field, and was still serviceable for as long as I owned the car. That was a total time of over sixty years for that top! It's only major deterioration occurred out in the sunshine during road trips, otherwise garage kept.
Sorry for my rambling, I just can't believe I sold that car.