I've posted up int the 'Parts wanted' section, but I don't hold out much hope that anyone will have one of these kicking about. I appear to have mislaid the half-moon shaped plate that retains the auxiliary shaft (part no 4129789, referred to as 'Plate, bearing' in my Beta parts manual). It will probably turn up when the engine has been fully rebuilt (i.e several years from now).
If anyone has access to this part and can measure the thickness of steel plate that it is stamped out of, that would be very helpful, as I'm pretty sure I will need to fabricate a copy to get the engine build moving along. I've checked in all the places where I squirrel parts away and it's not in any of them. This is what happens when an engine refurb takes an extended period of time, of course...