The front sight is, I think, unique. At least to Webley. I do wonder though whether they made it, comissioned it from a supplier, or - more likely? - bought a pre-existing item from (eg) Diana, HW, Anschutz or a company in their supply chains.
The rear is shared with the Eclipse and Patriot.
I too do not know whether the generic rear sight used with earlier and contemporary Webleys would be an effective replacement. But it would look wrong. My impression is that it might sit a bit too high, but happy to be proved wrong.
There are obviously many owners of air rifles from the 80s who have sold the rifles ages ago but still have the sights in the back of a drawer somewhere (or put them in the bin)!