The dealer did tell me when he looked it up, that it called for the 18T. The parts manual I have (think I downloaded it from here or from Zero) calles for 20T and the part number is stamped into the part. I prefer the 20T and that is what I ordered.
I didn't know there was a dongle for adjusting the speedo. Back when I installed the chain kit, Zero told me to take it to the dealer to get the speedo correct; they wouldn't give me the password to change it in the interface when I connect a laptop to it. I think someone did eventually post the password on this forum but I haven't gone back to try using it.
When I put it back on after the chain kit, I used the newer Torx screw (rather than the original allen head) and used blue loctite. It was snug when I removed it and there was no evidence of it being loose, e.g., paint scraped off.
When I get the replacement installed I think I will check the screw every few thousand miles just in case.