DJ - There is no "copyright" on manufactured parts. "Registered Design" is available for component design and manufacture, as a similar form of protection for intellectual property rights, as copyright is protection for the written or verbal forms of original thoughts.
"Registered Design" protection extends from as little as 6 years to as long as 25 years, depending on the country.
Not every part or component of mechanical items, is subject to Registered Design; this depends on how the manufacturer feels about his particular item design being copied.
The other item affording intellectual protection to parts, or complete products design, is a patent. Patent life extends from 14 to 20 years, again depending on the country where the patent was lodged.
There is absolutely nothing to stop any aftermarket manufacturer producing brand new Merlin parts and components, after this length of time.
The only thing they can't do, is infer that the new part is made or approved by RR, or use RR logos or registered trademarks. Any patents or registered design rights, conferred on any Merlin component, or the Merlin design, have lapsed long ago.