A wiring harness may just be easier to build yourself, there are places you can get the color coded wires and the wrap for bundling them together, and there are likely plenty of sources for wiring diagrams, worst case you can tear apart your old harness to make a diagram of your own. (speaking from the standpoint of someone who has looked into this for my '64 p1800)
Making other car parts is a different issue entirely, at least for the engine... most likely one that will stay in the realm of casting and machining instead of 3D printing.
An engine part often has very very tight tolerances, down to 1/1000th of an inch for mounting surfaces and alignments, something I don't see 3D printing replicating anytime soon, then to add to the fun engine parts have to hold up under stress from the combustion process, lots of heat and vibration.
Now, that's not to say it will never happen, there may be a 3D printer capable of laying down metal with strength comparable to cast or forged counterparts, with an accuracy capable of a mirror finish, but I'm not expecting it anytime soon or for a price that's less than the value of the entire car.
However, if you want to print cosmetic components of a car (rear-view mirror mountings, gauge faces, handles, etc) that is very likely possible now.