A realistic price is one that leaves buyer and seller happy.
Up and running is a whole different state of affairs. Puts a different complexion on things.
I for one had thought you were describing a project.
If all the spares are for the bike, my advice would be to sell them with the bike, I think it will add ££££ to the value, and after all there is not exactly a huge market for Radco spares.
I hope it has its original registration and a V5? that too adds value. Has it a full set of lamps? +++++£
As he has had the bike for so long I think a written account of its history, how he found it and when, what he did to it, any prizes he won with it, all that sort of thing is very nice to pass on.
I would consider two options; the site I linked to, and ebay.
Advertise it for what you'd hope to get for it, [hopefully after a bit more advice] plus agood bit more, ebay will take a big lump, and anyway you'll need a safety net.
Advertise on ebay with that fix price but as open to offers. That way it will be listed for 28days, and can be relisted free. You have no obligation to accept less than you list as.
If it doesn't sell its too dear, but I am sure you'll sell it.
At the same time put exact same ad on the classic car site.
If you have never sold on ebay I'll happily guide you through things to do and not to do.[ scams to be aware of]
love to see a photo or two.