It look slike green bike kit has that gear. Or you might find it on aliexpress if that's easier.
Bafang M820 nylon gear for replacement
www.greenbikekit.com
I'm also having difficulty getting SZZS to help with a dead m820. How old is your motor? Did they outright refuse the warranty?
Initially SZZS was very helpful with my DOA m820, but less than a year later it has died again, and they've just been stringing me along for the last two months. If this is part of a bigger problem with the company, we should probably get the word out so people can make an informed desicion before buying.