I replaced the standard Zippyy stick with this:
Seimitsu LS-32-SC Arcade Joystick
Very easy swap, it's pretty much exactly the same. It's slightly nicer feeling to use, slightly less clicky, and that particular model has a shaft cover and you can choose the ball colour.
To be honest though, it's not a huge difference to the stock joystick. I was expecting it to have a bit looser spring. Might have been more of a noticeable difference with a Sanwa replacement?
I have noticed that 4-way games, e.g. Pacman, are slightly harder to play now, it's difficult to get the exact direction - there's a restrictor plate, but for a picade you want to be playing a range of games so probably don't want to have to keep opening the case to change the restrictor for each game.
With the original zippyy stick, Pacman was definitely easier and movement felt much more positive.
But other games, with 8-way control, e.g. streetfighter, r-type etc. feel more fluid to play with the new stick.
So I guess it depends on what sort of games you like to play the most.
Things to bear in mind - you want an 'SE mounting plate' to fit right in without any case changes, and there are some models of joystick that use a 'H5P' connector rather than the little shoe-connectors for up, down, left and right.
An example of this is:
You can see on the image, this one has a different connector to the Zippyy original (so you'd need an H5P cable and rewire it into the picade controller board if you want to use one of those).
I stuck with a joystick with horseshoe connectors for simplicity.
I also replaced two of the buttons (the front panel ones) with these:
SEIMITSU PSL-30N-5W2 5V ILLUMINATED SCREW IN ARCADE BUTTON
Although these are screw in ones,rather than click in, there's still room. I don't know if there will be enough room to replace the side panel ones with screw-in buttons though.
The stock buttons I didn't think were too bad, but I wanted these illuminated ones, which are nice and subtle as the button bezel is illuminated rather than the actual button - I thought that would work better on a small cab. Also they are 5V so easily powered from a USB socket.
I am happy with the button replacement and originally intended to replace them all, but the cost is a bit prohibitive at £8.10 a time for these ones.
I'd imagine most snap in buttons will work fine, too.