Ebay has a limited quantity of DMR 20mm disc brakes hubs for 49gbp each . Just bought a pair . These and hydraulic brakes should haul you to a stop . Otherwise a pair of sturmey archer hub brakes would look a nice tidy conversion.
Only problem might be is the attachment of any middie drive into the bottom bracket shell ( overhang or rather under hang ) . Also beef up the rear triangle when installing e- assist as the extra help will try and shorten the wheelbase.
If it was me l would fit a motor to each half shaft. At low speeds in cornering the motors should be able to cope with the slight variation of current drawn . Also over bumpy terrain you will always have one wheel driving .
3 off 5s 5000mah 25c lipos and a charger will give 100 amps discharge capacity. charge each lipo separately to recharge them.
THESE ARE SERIOUS BATTERIES BUT STORE IN A DRY PLACE AND CHARGE IN A FIREPROOF BAG .
BTW A 4S 5AH ONE WILL START A CAR .
On your own head be it . Check what current they will draw . Personally l don't think so .
These are low current for fire alarms and burglar alarms and not suitable for motors of any type . I tried them in a rc boat for enough time to get to the middle of the pond and STOP.
I would think 7ah and 4 x 12v in series might work . But car batteries of 7 ah have only a useful current drain of 15 ah .for a short period . I used them on the original red trike and got 6 miles on a 500 watt motor . These small motors draw a lot of start up current dispute their size.
You will need a hefty power supply to run each motor off load . The motors using DC to the controller and a modified " 3 phase " output . Not easy to duplicate without a battery . Then you will also need a charger . Then you will wish you bought it all together when the item goes zero stock ...
The change over switch would sit between the controller and motor . We have done this on some of the RC boat projects.
On initial setup you might need to swap over two of the wires to get both motors running in the same direction.