Evening!
Is it here I point out that the US and US certification isn't the world's?
While aero engineering is a remarkably standard system globally (I know, I know, but bear with me!

) each country can and will certify and regulate to its own criteria, and will also choose to validate or endorse other nation's validations.
While I'm not a 'hands on aircraft' guy in the warbirds biz, a couple of interesting examples come to mind.
There have been examples of aircraft built in the original factory in country 'R' and shipped to country 'E' where the paperwork isn't acceptable. As there is no means of validating the quality of the metals used, the aircraft and owners are in a bit of a bind.
The weld on the wing strut of the replica 'Spirit of St Louis' Ryan replica gave way in the air during a display in the UK, in 2003 and the pilot owner was killed. The aircraft was based in Sweeden, Estonian registered and built and the British accident report (AAIB) highlighted the concerns over the build quality and issues over the lack of a handbook and lack of a walkaround procedure. As a result of this sad accident, there have been questions over accepting aircraft for display from overseas coutries (again) as issues of what is acceptable in Estonia, Sweden and the UK (as well as, say. the USA, Canada, Australia and NZ) is different. And no, it's not an EC question in this case.
The accident report makes very interesting reading:
(
http://www.aaib.dft.gov.uk/cms_resource ... 026654.pdf)
To try and get back to the original question, not being an engineer, I can't give a definitive answer, but is it worth saying that in warbird rebuilds, the UK model is that parts need paperwork, the work needs to be 'signed off' by a qualified engineer and you go form there?
Hope this helps.