Hi Funsize.
The Morini Riders Club Technical Guru, Paul Compton, did a YouTube video showing the re-assembly of VHB carb which is well worth watching. The needle doesn't get a mention until 20 minutes in and it doesn't show how to remove or move the needle.
Still, it is quite simple once you have the slide separated from the cable and spring. You just pull out the flat metal clip and the needle drops out. Getting the clip back in on the required slot is a bit tricky as it is hard to see, peering through a small gap.
Another good point Paul makes is about the two small bolts that hold the top cap on. These have a reduced shank between the thread and head. There is a corresponding thread in the cap so the bolts thread through the cap and then become loose and captive. If you unscrew them completely they can fall out and disappear under the workbench never to be found again. Don't ask me how I know.
You shouldn't need any gaskets as a matter of course although there are a number of 'o' rings which can become hard or crumbly over time. The seal kit shown in the video is the 52510 set, available from Eurocarb.
https://www.dellorto.co.uk/
Here is the video. Good luck.
https://www.youtube.com/watch?v=oKBdvmGYWQM