>>1335861>transmission upgradeYou would have to change a bunch of other things to make it work. That ‘mid drive’ idea might work great in a bicycle, but it was never designed to work in an industrial tricycle so it and the trike would have to be adapted to each other. Thus we arrive at our old friend: custom welding and machining. You would also have to come up with some sort of brake, because the derailleur stage is not compatible with coaster brakes and does not include one. Better rear brakes would not be a bad idea but would be a significant expense to design and fabricate as well. All of this is vastly beyond you budget of “effectively zero”; it would be cheaper to buy what you really want as a new mass-manufactured product.
