classic motorcycle forum

You'd have to ask why a "professionally restored" bike would be barn-stored for ten years, though just because it has an age-related plate doesn't mean it was never a bike "in one piece"; I had a T100SS which was also tagged with the "built from parts" on the V5C. I think it something the DVLA did back then to cover themselves when issuing an age-related plate.
Funny thing about B31s is the values don't seem to vary that much between useable and needing a rebuild. I was watching two on Ebay two weeks ago; one was a late incomplete pile of parts brought in from Spain and missing most of the small parts, brackets, pegs, clock etc and the other an early rigid with no lights, ally guards and clearly had been a field bike, but was allegedly running two years ago. Both went for around the £1500 mark, but it's not unusual to see runners go for not that much (relatively speaking) more.
Don't want to sound cynical but "restored and never used" sets the old alarm bells ringing, but if it does polish up well someone will surely buy it regardless.

The timing cover looks to be off a 500, the flat-backed headlamp is off one of the OIF models,and the rest will need more careful examination.

+1 for the mag, but an Amal Mono should be a direct replacement, as should a later Concentric. Hard to see how the present carb could be at fault with those symptoms though.

In some respects the older the less sellable.
Although many would argue that belt-drive/single-speeders/etc are perfectly useable, the reality is they're not, especially in general 21st century traffic, and so a bike from the 1930s with proper clutch gears and oil pump would be better if someone actually wants to ride occasionally rather than just have a pretty ornament.
Not to denigrate your bike in any way, just pointing out that the old boys who could appreciate a Radco would also appreciate an easier ride..

But the detent is only there to keep the camplate in position once a gear / neutral is selected, not to control its movement

Not really. Take the detent plunger out (not possible on all 'boxes admittedly) and it's hard to find neutral as the box goes from 1-2 or 2-1.

I hope you're a young man....'cos if you're starting with a bare frame and engine only you'll be needing to live a long time to actually get to ride it.
Unless you fit generic parts to get it going, but then of course you may as well have bought a BSA, Triumph or whatever in the first place!
Good luck with it though.

My guess would be the indexing is out. Read the manual to see where to position the selector when fitting the cover.

Oh yeah, I remember that now.
On the same site they're currently discussing to the nth degree how to remove a head race. Dremels, multiple pullers, strange contraptions and turning up a threaded bar.
How did anyone get on before the days of t'Interwebby, and advice from random school teachers and retired opticians?