If the shaft groove is so badly worn that the clip falls off, you can try making it a bit deeper with a "jewellers" edge file or thin saw blade. Drilling and tapping the shaft end for a retaining washer and screw is an option, that would probably look better than a cross drill and small split pin? The part with the pin (50641-377-000B) is basically unobtainable new, and used 400F parts are uncommon - plus their quality is not reliable. You can try Ebay...