As of yesterday evening, while there was enough light to take pictures:
The new shaft turned up earlier than expected so I swapped it in and got it bolted up. You can see the spacers between the rockers instead of the springs that are normally there.
She was running late from work so I cracked on and did the valve clearances and bolted everything back on. I pulled the plugs to make it easier to turn over by hand and it was also a convenient time to check the mixture:
They don't look too far out to my eye. Looks a bit lean, which ties up with the flat spot when you open the throttle, so hopefully the different emulsion tubes will clear that as the ones in there are for progression on a larger engine. I don't completely understand what they do, but you match them with the cc of the engine.
I got it fired up and took it out for a spin. It's super quiet at the top end now, so I'm pleased. In my experience, these cams usually result in a noisy top end, but I'm thinking now that the wear in the rocker shaft has a lot to do with it.