I'm surprised no one did this years ago. It is a very simple part to make for a machinist, and there is an obvious demand. These restrictors never really wear to a significant degree (the original ones don't anyway; I assume the reproductions won't either) but they are often missing, or the wrong type for the game you want to use the joystick with. Apparently a lot of operators would turn a 4-way Nintendo joystick into an 8-way simply by removing the restrictor and calling it good. While this does allow the stick to work as an 8-way, it makes the stick very sloppy feeling, and also allows more stick travel than the stick was designed for, which often results in broken springs, broken microswitches or bent microswitch actuators. It also mangles up the metal of the base around the hole where the restrictor is supposed to be, because the base is a thinner and milder steel than the restrictor, and wasn't intended to deal with impacts from the bottom of the shaft during usage.
I'd like to see someone reproduce the ball tops (they are threaded, right?), the short clear plastic sleeves below the ball tops, and the shafts too (preferably out of stainless steel), for cosmetic restoration purposes. The ball tops are sometimes missing, or more often, simply beat up in appearance, and the shafts are often rusted/discolored due to peeling/bubbling chrome.