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This is the registration page - you need V5c and the some kind of proof - a homolgation letter is the listed, I've no idea if you'd be able to get in touch with a human to find out if there's a process for where that's not available....
https://tfl.gov.uk/modes/driving/compliance-registration/before-you-start
Mine was a late 90's car (non turbo - is it true that turbo's are generally cleaner?) and the NOX on the homologation letter was well within the limit. Love to hear if you managed to do it!
This might not be aplicable, especially since it's Saab and records might be tricky to get, but I managed to get a non euro-4 car exempted from the ULEZ.
It took a homologation certificate (teh bit that might be impossible) showing its nox emissions and then a submission to tfl for exemption. Took about a month to gather all the bits.