Here are some links to the sellers.
French bicycle shop.....
Italian bicycle shop...
I got s set of those tyres just 2 weeks ago from France.(37 - 642 or 700A), they are the Vee Rubber brand, made in Vietnam.
They are available from one French Ebay seller, also an online bicycle shop based in France, and also online from Italy.
I can't help re. the maximum box size that they allow, but I know it can be a problem from past experience.
My example had the wheels, bar/stem/levers combo and saddle/stem removed from the frame and was fitted into a standard cardboard bike box that you get from a retailer.
The chainset was...
They were only available for imperial size wheel/tyre combos.
You would have to measure the overall diameter of a British 26" wheel (with the tyre fitted) then compare it to a 559 wheel (with the tyre fitted) and see how far out the reading might be.
Could be a 'Ural' as stated above or a 'Dnepr' (solmetimes sold under the 'Cossack' brand here in the UK).
Both are still in production and are based on the WW2 era BMW R-71 which the Soviets acquired the licence to copy in 1940.
Available in 1 or 2 wheel-drive versions.
The military styled...
My example was shipped by the Italian national postal service, 'Poste Italiano', then delivered by our national postal service, 'Royal Mail'.
The bike shop in Bologna had no issues and it cost around €200 on top of the price of the bicycle.
I realise it may be different shipping it to the UK...
I think the period repairs are all part of its charm and history; but patina and its story would mean a lot to me.
I'm assuming it originally had larger wheels?
Could you not refit some and maybe ride it?
Their track racing squads, alongside the then East German teams were pretty dominant in the 1980's; that requires some serious metallurgical skills to produce lightweight componentry able to cope with the stresses induced under these conditions.
To be fair though, their cars and motorcycles...
They do keep popping up from time to time in Russia and the Ukraine. The "CNOPT" examples usually look pretty rough though.
The Takhion pursuit bikes (and road TT bikes) with the fork mount bars are fetching crazy money!.