By the notch on the arbor do you mean the bit that holds the mainspring? I am sure someone will be along in a bit to help on this one but be a little careful with the replacement mainspring, they can be a lot stronger than they used to be, I think I am right in saying you reduce this thickness by a certain percentage to overcome the extra power but this is not gospel, wait until another member gives confirmation on this. I wouldnt know how to go about repairing the arbor either, looks like I will be learning something new here too 
I would be very interested to hear how you get on with this as I have just started on my first 400 day clock and I have another 3 to play with after this one!
