There are plenty of classic specialists who have created their own blueprints and then had parts manufactured. Sometimes, it's quite a hand in glove affair (such as British Motor Heritage, which produces 'new' MGB bodyshells), while other times it's very much one man and his shed operation. One thing that's definitely worth looking at is 3D printing, which allows you to run off an item as when you need it - rather than order a whole batch. I know a lot of Alfa specialists swear by it as parts for those cars have become incredibly hard to fine.