You have patents which are valid for 21 or 25 years - they expire, and anyone can buy them. You pay for the patent, and therefore you can control the production and distribution. You can even on-sell the patent.
Copyright does not really exist on pinball parts, that's the job of patents. Copyright is more for documentation such as manuals, schematics etc.
A backglass is more of a patent, but I'm not sure. Under the "right to restore" you can produce your own backglass BUT according to the law, you may have to destroy or forfeit the original.
You can ask permission from the patent owner to produce your own part - Gottlieb LLC are EXCELLENT with this regard. Bally Williams would be more approachable these days. For the EM machines, Gottlieb LCC still would police anything made without their consent. It is unclear about Bally or Williams EMs. I was once told that Williams games prior to 1982 were not under any patent. Same with Bally.
Having said that - If you picked out a Playboy or KISS and wanted to produce a plastics set for resale, you would have a cease a desist from the band or Hugh Hefner's estate.
It is VERY complicated, and I've heard conflicting stories - everyone has their "own version". It is a little more "relaxed" these days, but you can't blame the patent owners for protecting their investment.
