I'm certainly no expert, and I can't answer the question, but there's a couple of items here I'm pretty sure are wild geese...
John Dupre wrote:
It is possible especially working on US registered aircraft in the UK. I believe that a number of warbirds operate there on US registrations.
Not any more, I understand, The CAA (FAA equivalent) clamped down on that in the 1990s, IIRC. (Happy to be corrected!)
CrewDawg wrote:
Now for working on warbirds over there, it's probably a little easier. I would suggest contacting the Imperial War Musuem, Duxford and asking. I'll bet that they will let you come over there and work and they will have someone sign off you're work, similar to our repair station's here.
The IWM doesn't operate an active aircraft at all. The flying aircraft at Duxford are all privately owned and operated by companies using the museum as a base. Therefore you won't get anything useful from the IWM except perhaps a referral. The organisations operating warbirds are the Fighter Collection, Old Flying Machine Co, Sally B Ltd, Historic Flying Ltd and the Aircraft Restoration Co among others. They may be interested in discussing options and what you or they can do to make it legal and viable.
In the absence of anyone who knows the answer here (so far) contact the Civil Aviation Authority, I understand they are the governing body in question.
http://www.caa.co.uk/
I think this page might help:
http://www.caa.co.uk/default.aspx?catid=137
Canada needs compatibility with the US due to proximity, trade, likewise across Europe there is a much greater standardisation. I have no idea, but I'd make no assumptions of Transatlantic compatibility.
Also:
http://www.alae.org/index.php?p=49&pp=0&title=About_Us
This may have some useful contacts /advice:
http://www.questonline.co.uk/articles/a ... neering__1
There may be spelling and vocab English tests for Americans

(Most British engineers will know your lingo - they'll have terms that you'll probably not have encountered before, unless you've worked in an English-English environment). Don't ask to be paid in Dollars or Euros, they're very proud of trying to pile on their Pounds.
I suspect if you were working as an unpaid volunteer on warbirds, you'd not have signing authority anyway as a rule, but relevant experience would probably be of interest, and if there's a question of converting paper, to make you 'more useful', they'd be able to advise.
I'm sure someone better qualified can answer much better, but in the meantime I hope that helps!