Hi Will, I took a look on CMSNL - they list the front stop switch assembly for 1977 XS650E, part no 1A08398030 which includes the switch body, the wiring and that clip for 54 Euros. It might be possible to find a source for the clip itself but maybe just use a small Jubilee clip?
If it's just the rust, I recently discovered that leaving the part in vinegar overnight softens the rust which wipes off leaving the metal looking much better. Well worth a try for smaller parts which sometimes come up looking almost new. Now keep a bottle of vinegar in my products cupboard.