Due to the design of the High Standard Model 200/Sears 20, no magazine extension or sling plate can be installed.
The only option on extensions would be to replace the entire magazine tube with a longer one, but I doubt any are available.
Since there is no screw-off magazine cap, you can't install a sling plate.
Here's an exploded view of the Model 200
What you can do to install a sling swivel is to replace the magazine band screw with a longer screw with a sling swivel hole drilled in it.
Most any machine shop can easily make a screw to fit with a large head with a hole drilled to accept a detachable swivel.
Another option is to drill a hole in the front of the magazine cap and install a sling swivel stud. Uncle Mike's sell the studs and a nut to retain it.
Most any brand of ghost ring sights can be fitted.
The receiver will need to be drilled and tapped.
For a front sight, you can have a sight base brazed on, or you can use one of the clamp-on sights from Brownell's. These are strong sights that simply clamp on and don't ruin the finish like brazing does:
For repair parts, look here: