A lot of old cocktail games from 79-82 had 2-way joysticks which were basically IDENTICAL to the Defender Stargate joystick. They can be found without too much trouble, and the player 2 stick is usually in like new condition. I have a couple of these, and they will take the Defender repro knob and handle if you drill one hole in the thing slightly larger.
Also, there is what I did with my Defender. I simply used a Wico microleaf, cut a slot in the wood to restrict the movement, also turned the side microswitches around backwards to further doubly restrict the movement.