I have been working on small power equipment nearly 50 years. Just closed that part of my shop and retired couple of years ago, but still doing some "cash & carry" local work here and there.
Couple of things that I REFUSE to get involved with are Chinese P/C's and replacement carburetors. If and when they are available use OEM and save yourself the aggrevation of dealing with sub-standard components.
Folks go that direction simply to save money, but you really aren't saving anything. The failure rate for aftermarket carburetors here is 100 percent. Most don't work right out of the box. Even when they did we'd get a couple months out of them then they would fail. I'd add here that many didn't even work well and never performed like OEM even if they got the equipment back in service.
I found this out early on messing around with cheap replacement parts. Carburetors top the list for items that get repaired or replaced on chainsaws, string trimmers, and other seasonal equipment. This simply happens due to the reformulated fuels many folks put in them and they fail to drain them after done for the season and leave the fuel in them for many months. That NEVER goes well.
Anyhow, in any and all cases obtain a genuine OEM carburetor kit and rebuild the original. If you are doing certain types of carburetors you may also need to obtain a gauge to measure the metering diaphram height as the are specific to certain carburetors and must be set to exact specifications.
Back on topic the Walbro HDA 268 carburetor is an excellent unit. 99 times out of 100 it will only need a genuine kit installed. Once in a while you'll get one that needs a new main nozzle/check valve assembly. I keep a few of those on hand here. The replacement nozzles are grooved so you don't have to worry about alignment but you still have to keep the depth at the factory specs so protrution into the main bore and alignment with the fuel supply hole is where it should be. They really aren't fussy and I've never once seen the need to replace one as they are very well made and easily last the life of the saw with maybe a few cleanings and kits installed along the way.
I'd add here that IF you decide to replace an HDA 268 on a CS-590 or 600P the larger HDA-316 for the CS-620's is a direct replacement. They are about $80-90 new last time I bought one.