I presume the new screw would have an extra large cap for the oversize hole. Will the new screw be flush with the sight tang when installed?
I would tig weld the hole and re-drill it to the proper size, and counter drill it properly for the original correct screw to sit flush. Then the factory screw that came off with it will still work,
Rear sight leafs are not hard to find. Depending on what model and barrel length the rest of the sight might be used for, the w/o sight leaf might need to be changed anyway.
I would not junk it, too easy to fix or someone else to salvage and fix it. I don't junk any old/obsolete parts. There are too many people out there these days with the skill to fix them. When I need a part but can't find the correct part at any price, fixing one starts looking pretty good.