IIRC, someone in another forum years ago lost a limb in an accident. He actually fabbed up a left hand control to shift gears. It was basically a throttle type twister, but for the other hand. I don't remember how it all worked out as that's also the clutch hand, but given that the gears ratchet, I could see that working. I think he lost below the left knee and was able to ride again.
IDR the exact design, but if you can imagine one of those cruse control levers on the right, being used on the left. So you have a twist control, but you control the twist with a lever instead of the grip. That allows you to thumb your gear shifts and work the clutch at the same time.
Motorcycles are made generically, I know a guy at 5'2" that rode, he had to make sure that he stopped on one side as he had to slide to that side or he would fall down.