[Arran]

I don't really see that as a concern, for one thing the originals, even well kept ones, will have signs of aging and wear. If I am not mistaken they may also have cabinet shop markings such as stamps, not to mention paper labels. To be honest though, unlike the brand Z "Waltons" models there isn't much profit incentive to commit fraud by passing off one of these as a original, particularly when you add up the parts, and matching the correct interior finish, external finish, dirt, aging, most fleabay sellers don't have the know how or the motivation to pull that off.
In fact I doubt whether you would get any more or less for a 90 with a good repro cabinet and finish as opposed to a restored original. Everyone that I have heard of who bought a Dick Oliver 20, 70, or 90 cabinet did so because they had a spare chassis and speaker, or had a complete set with a wrecked cabinet.
Regards
Arran