Hi,
I agree. The provenance on most WW2 aircraft related artifacts is very hard to prove conclusively. Without specific and detailed evidence, many artifacts have little to prove their past history.
It is quite rare to find parts that have unambiguous identity markings, but even then only a rare few are serial numbers that can be proven against records. Most parts of an aircraft
are not marked with the identity of the aircraft that they were fitted to.
So, I think you are faced with some problems, but they are not all impossible.
I presume that you have a particular sphere of interest in the WW2 aircraft and their relics, possibly including a campaign or theatre? I suggest it would be wise to start
with what you have some strong interest in.
As for finding the relic(s) to work with, this is tricky. If you buy items with a "provenance", you will have to decide how good that is and carrying that "provenance" forward will depend
on it's strength. IMO, just saying that it is a woofle-valve from such-and-such has little value even if it is actually a woofle-valve, it is unlikely to come from such-and-such unless there
is evidence to support that.
Possibly, with dedication, you might find relics that do have strong provenance, and that would be the best situation.
You might find that you can become associated with an aircraft recovery group. This may help solve the provenance problem, but I suspect that many of those groups have strong self interest in relics and you might have to work hard to get involved. Another possibility might be to properly approach some of the local aircraft museums and see if they have any items that they are willing to sell or dispose of. You might be lucky, often they are short of funds and some have worthy relics that are out of their main theme or surplus to need.
Lastly, I would always advise that you take special care with the identity of items being exactly the item that people say they are!
Best of luck
Eng