Hi, your best method is to remove the sight (drift out from the left as looking down barrel and replace from the right), and use tig welding to build up the sight. First you do a few small runs to get the blade hight somewhere close, you then shape this with a dremel and files, then you do a larger bead at the top and re-shape the bead, with practise it takes an hour or less, and the finished result is as good as the original in appearance, and better in respect that the sight blade is now slightly more malleable and will take a knock without snapping like the original casting. Also the base is original so no re-fitting woes as you often get with pattern units!
I use 0.8 mig wire as a filler, with the copper coating rubbed off with scotchbrite, an alloy steel like bostrand 41 is good, but mild steel a18 works well too.
An example in various stages;
This one had a larger bead than original when finished, or you can make it barleycorn or any other profile offered by BSA originally.