I rebuild carbs very regularly. I use and recommend Keyster kits.
As a minimum you will need bowl gaskets and o-rings. Often you need the float valve. Once you need gaskets, o-rings and float valve, it's often cheaper to buy a complete rebuild kit.
You may also find that on the z900 carbs you will need new fuel tees. there is nothing wrong with using the Keyster main and pilot jets.
I would always prefer to keep the OEM need and needle jet - I'd only change them if they are worn.
You won't know what you need to replace until you strip and inspect.
Also you sometimes find poor running because the sealing rubber on the end of the plunger of each choke is degraded. I use Rob Meggit for this who can replace those rubber seals.
I sell viton petrol proof bowl gaskets that I have made in the UK. These are a fit an forget solution as they don't degrade like paper ones. (Some of the keyster kits have rubber based bowl gaskets included, but it seems a bit pot luck.
In summary, sorting a set properly takes quite a bit of time, effort and money.
Do get in touch if you need any help or advice.
You can see some of my carb work here:
https://www.facebook.com/airevalleyclassics