Adapt a wordpress 3.1 customization to wordpress 4.2.2

I would like this exact mod applied to my blog: [login to view URL] It allows me to select a specific user when creating a post, and only that user can see the post. I am using "X Theme" for wordpress. I would then like the Recent Posts widget modified to exclude these posts from users not selected to view them (regardless of category). Will give a bonus if the above mod can be slightly altered to be check boxes instead of radio buttons (to select multiple users per post and save the array of users). Would like all changes to be unaffected by theme updates (although I don't know if theme updates affect the new post page or widgets, just specifying)