What I mean by "they'll get it through" is ...
Not all bikes pass on the test first attempt so they'll make adjustments/modifications so it passes.
Call them and ask for advice on your particular model and they may have info.
This is what I did before I bought my 2000 Honda CB500 , I called them and asked if I would have trouble passing the ULEZ test and they told me I won't as they have passed plenty in the past.
I've yet to take the test as it's not needed until October , I plan to make an appointment in the summer .
I've got a few customers with 80s era bikes that have passed the test so Personally I think you'll be OK with a few tweaks.
Oh .. for your info, they are the only company in London that are endorsed by TFL to test motorcycles and will exempt your bike there and then if it passes.
Good luck....