Sights? The only aftermarket ones I can think of are the Mojo sights.
https://www.mojosights.com/sample-page/n/
I've tried the Mojo rear sight on an SKS and didn't really like a rear aperture that far from my eye. Some people like it, but I didn't.
I've also used an S&K scope mount on a Swiss K31. It replaces the rear sight and you will have to get a long eye relief scope with 9" or less eye relief to work. Or use a red dot with unlimited eye relief. You will also need to work up some sort of cheek riser if that aftermarket composite stock doesn't have one built in.
http://www.scopemounts.com/index.html?main.html Dig around on the "Ex-Military" drop down menu. Look at all three of the items in that menu. The Mosin Nagant is in there.