dracb - I am sorry I cannot help you with the tang sight, the only one I have is far too short
Possibly mount a thin block to fit the existing screw holes (countersunk) and fit a suitable sight to mount on that

Many thanks for posting such a superb example of Westley Richard's 1869 patent "Improved Martini"
(they were competitors to Martinis, slightly earlier, only sharing the hinged block) with best quality engraving

The .450 No. 2 Musket is arguably a much better cartridge than the .577/450 and is good to load as well as taking most game under the right conditions

I hope you get the right sight for your eyes (mine are also getting old!) and good luck

TH44