Peter wrote:
There are various documents such as the cert of airworthiness, and
(for an N-reg in Europe) the FAA radio license, which some say need to
be carried in original.

Can anyone point to a reference for this? I can't find it in the 2005
FAR/AIM.

In the US there is no requirement to carry a radio license. If there is such a
requirement where you live, the requirement must originate in your country.