on vichan thread URLs were formatted like this: https://boards.guro.cx/art/res/7184.html
on koko its like this: https://boards.guro.cx/art/koko.php?res=7184

so if you have any old bookmarks that 404, just do https://boards.guro.cx/art/koko.php?res=X
replace 'X' with the thread's number (the '7184.html' part but without the '.html') and change 'art' to whatever board the thread was on

BONUS: the res parameter also accepts reply numbers, so if you remember a post's number but not what thread it was in on a board, then do something like https://boards.guro.cx/dis/koko.php?res=24040 and it'll redirect you to it