Here is the outline of a two speed belt or chain drive idea that is very simple with only two belts or chains between the motor and wheel. The idea is similar to the two speed belt drive on the evo/uber petrol scooter, and similar to Thud's two speed box.
motor drives through both belt/chains motor rpm is reduced around 2:1 in the first belt/chain that is very short with further reduction through the final drive belt/chain, ratio dependent on what you want for the final reduction, wheel size and speed requirements.
second gear: drive is through the final drive belt/chain only, with only one stage of reduction.
device comprised of motor shaft, jackshaft and two belts/chains, the final drive belt/chain path wraps the rear wheel sprocket, jack shaft sprocket ~180 degree wrap, tensioner 180 degree wrap, motor shaft sprocket ~180deg wrap then back to the rear wheel.
shifting: there are a number of options here, a single clutch (friction or dog) on the motor shaft to final drive belt/chain sprocket and a freewheel/ one way bearing on the first gear jackshaft to final drive sprocket this allows the motor to freewheel in first gear but be locked for regen in second.
second option is a double dog clutch on the motor shaft where loading is lowest with a neutral position allowing disenguagement or regen in both gears this is my preferred arrangement.
This design allows the first gear chain/belt and dog clutch mechanism to be encased and allow good lubrication of clutches and first gear chain if used and only two belts /chains to be used, with only the final drive in the open with dirt and crud, the use of a belt here is probably preferable as it is likely to be on the left side with the rear disk brake but i would bet that with two gears and regen the back brake is never used unless something fails.
If I build this it will be likely all built on to a lengthened swing arm using an unshelled BPM 6t motor I have already (50kv 1500w cont ), but there are plenty of suitable motors for this type of setup.
Let me know what you think people.