LunaCycle has you covered.

Thanks LunaCycle for having all the parts to repair my BBS02. It was the exact cover (gasket included) that I needed but too bad that Bafang makes these covers out of plastic. Several chain pop offs eventually cracked the plastic cover. I didn't notice the crack because it was on the bottom side. The crack allowed dirt to enter the gear housing and eventually sanded the gear teeth down to the point where some of the teeth sheared off causing more damage to the large gear and pinion gear. If your chain ever pops off, make sure to check that plastic cover and adapt your bike to prevent future chain pop offs.