The best improvement you can make to the Bafang BBSO2B is to get it reprogrammed by Venture Scooters. The next best thing is to fit a Lekkie front chain ring. These are called Bling Rings & are a serious piece of equipment. They are made from Aluminium using the latest tooth shape technology (wide/narrow) & are as hard as nails.
They are lighter, stronger & will stop your chain falling off when using on bumpy surfaces. They also give you a better chain line which is really important when doing these conversions. There are a range of sizes to suit different types of bikes & types of riding.
The 40t chainring is the smallest ring you can fit to the BBSO2B that still gives the correct chain line. It requires a custom outer motor cover which is included in the kit. The best chainring for off road riding & climbing bigger hills.