As you are probably aware the carbs are specific to this machine so are pretty rare but I have seen them in auto jumbles as with the magnetos.
But it is a waiting game.
As for the the tool box lid, Is it not possible to make this?
I have enclosed a few photos. Looking at the lid, it is basically flat with a lip round the outside and on the bottom lip there is a ridge to hold in to the bottom of the lid to the tool box and a hole drilled at the top for the bolt fixing.
I do know of a couple of owners who made theirs, one out of alley sheet and the other out of plywood. Both of these lids look good and when on the tool box you would not know any differant.
Even if this was done only until you found a lid.
I must admit I have found a tool box at an auto jumble but it did not have a lid.