I think the screws that hold them on are 4BA. The trouble with holding a scope rail on like this is that you are putting something with a flat surface (the bottom of the screw) , through something that is rounded (the cylinder) and quite thin. If the screws are a fraction too long , then they foul the back of the piston and the gun wont cock. On one I had, I marked where the screw lined up and then filed a groove on the bottom of the screw.
If you can find a scope and mount, I think it would be possible to remove the scope and fit a modern cheap one in the mounts.
Another option is that some people on here make up a rail out of a bit of tube and fix them to the cylinder with glue (tiger seal?) , which would give the option of more modern / better scops.