Jay Leno has been making parts for his stuff for years like this.
Exactly, but maybe I should have clarified myself a little better because like I already mentioned there are already services out there which are costly and limited to those willing to spend the money.
What will be neat is one day when there is an accumulated digital library of all the various parts to all these various old vehicles and all a person has to do is read through a catalog listing for the item they need and then print it on the type of 3-D printer that fits the job. (3-D printers can use plastics or metals)
Part of the issue now is that in order to print anything on these printers you first must tell the printer what you need printed, meaning it has to have the dimensions, design and correct materials before it can do the proper job. This means you have to have a 3-D scan of an original item or the details of it already in some digital form so the computer knows that it needs to create.
In a rare one off car like what Jay Leno has the chance of a part being needed by many others is just not likely, but say you need a certain badge in new condition or a piece of new hard to find Cadillac trim I would think such items could be produced with enough demand that it could make such a time investment into scanning them into a data library worth while. You then sell the item as a digital blueprint and not actually the item. The buyer simply pays for the design had it emailed to them and they then load it in their printer and print it.
In regards to pro cars look at the items for a proper restoration that are often needed, such as new name badges (Miller Meteor, CB etc) lights and lenses, trim, interior components such as cabinet nobs, locks, hooks, rails, and the many other little chrome or trim pieces. I'm sure its not long before we see this stuff available as I know people in other circles that are actively working on such a businesses model to bring these types of 3-D printing technologies to the automotive restoration circles and masses. the future of this really looks neat, but with any technology the first several years is a huge learning curve.