is there a simple way to cut a restrictor channel to force a 4-way to only move 2 ways?
I did exactly this for my modular CP.
There are two ways to go about it.
First, you can drill two holes, and then jigsaw the slot between them, and sand it smooth.
Second, you can drill an entrance hole, and then rout the slot using a guide.
I have extra joysticks I could re-purpose as long as I can replicate the height and throw properly.
The joystick you pick will determine how well this works.
I lucked out on this point because I picked up one of the really small Taito 4-ways in a lot buy.
They have a very short stick, very small balltop, and 1/4" shaft on them.
When bottom mounted, and routed slightly into a slotted CP, they very closely mimic the height/feel/throw of the original Defender sticks.
The one modification I made was that I cut one corner of the joystick base off, to allow me to properly position the reverse button in relation to the stick.
This modification will work on any joystick with similar construction to a Happ/Wico joystick with no problems.
The only problem I see with using a Happ stick is the throw.
If you want the stick low to the panel, like the original, it's going to have a very long throw to it.
This throw length is going to be critical because you need to move up/down rapidly, and repeatedly.
You can adjust a leafswitch stick so that it will actuate immediately off center in both directions, allowing very quick up/down movement.
You can't do this with a microswitch stick, so you are going to have some control lag, based solely on design of the stick.