Well, it looks like the joysticks that were in the Deco cabinet i had are a bit different.... but the principal is the same on your missing piece.
You may have some luck going down to a hardware store and looking for a metal "spacer" of sorts that will fit. Looks like yours has threads for a nut to hold that actuator on as well. (mine just has a groove for an E-clip)
The spacer just needs to have an inside diameter to fit the shaft and have an outside diameter to just barely touch the switch actuator levers. And the length will have to be where it seats all the way up in the joystick and comes out to where the flat spot is where the threads start.
Heck, a machine shop may be able to wittle you one up also.