Nick - someone with a small lathe (e.g. someone that makes scale-model trains or boats) could turn out those brass collars easily out of brass round bar and you can buy circlips. Only the diameter of the peg that goes through the bell crank and, maybe, the hole for the rod are at all critical. The larger diameter and thickness of the collar and the grub screw size can be varied, within reason.
My lathe is out of commission just now, but I can think of somebody on here that

might be able to help out here? T.
PS - You could even adapt some old Meccano brass collars, if you are that handy?