Found an interesting rear caliper from an overseas supplier. Conversations with the E-Motorbike community have informed me that this caliper would fit the QS motor hub well, and after an evening of fitment I found that the community were correct. Hurrah! The rear swingarm needs some small modification, but i think these will be minimal. My only concern it the size of the caliper. However, most braking is done with front calipers in reality (70f-30r) and regen braking minimises wear on vehicle brakes on EVs so this caliper may be sufficient. Only testing will tell.