I just watched a fun video of a guy bringing a 1978 Datsun 280Z up to date. He kept the existing engine but upgraded it with modern electronic ignition and computer control. As part of all that, he fabricated various parts in plastic by 3D printing, but then had the final versions made elsewhere in metal -- some by a CNC shop you send CAD files to, and one by a manual machinist in Switzerland with his own Youtube channel. The latter has to be case hardened, which the machinist did.

The 3D printed plastic parts were very useful to prove out the parts' shapes.

https://www.youtube.com/watch?v=jZ38C-M3tyk -- engine upgrade

https://www.youtube.com/watch?v=D2bxsEyYdo0 -- My Mechanics making the part