Yes spotted the ball joint, very good till they fall apart as happened to my son half way round a trial section last Sunday, suddenley found the accelerator pedal was on the floor and no revs.
I see now, it is a shouldered bolt, no idea if you can buy them off the shelf, probably cost a fair bit to get some made. Using a collar on a standard bolt sounds like the best option.
I assume you have checked for a spark to rule out an ignition fault. British Telecom had most of their portable pumps and generators running on gas, this avoided the need to carry petrol in cans and most vehicles had gas bottles on board already. It might be worth speaking to Meetens as they are Villiers parts specialists.
Photo is a bit blurred but I am guessing it is a square headed coach bolt, ie it has a smaller square below the head to stop it turning when fitted in a square hole or in a slot and has a whitworth thread. Looking around this link might be useful https://www.myersblacksmiths.co.uk/#/rivets-bolts/4565523902
The Amal carb in the illustration I copied differs slightly in that the float chamber is under the carb and held by two screws. Your carb has the float chamber on the side and I think you will find the original top would have screwed into the float chamber. The thread may have got damaged and hence the replacement top.
Seeing the photo from busman it does look as if you have a non standard top to the float chamber. I am sure a photo giving a better view of the rubber bung and surrounding area will confirm what it is.
The pool of knowledge within the club is vast and unfortunately some will be lost over time unless it is passed on, either by word of mouth or written down, (puts hand up with guilty look on face).