Hey guys, I know this is not a robot but I could really use a second set of eyes on this one...
Stop being a pussy and saying it's not a robot! this is a robot site and your a robot builder. Make a high power servo from a windscreen wiper motor and connect a pot to the handle bars. It's an electric bike so you have the power!
Most electric bike motor controllers will not allow the motor to work below a minimum voltage as it can damage the batteries. As a windscreen wiper motor works on a lower voltage than the bike motor your steering should continue to work fine even when the drive motor refuses to turn.
I'm assuming your electric bike motor is at least 36V. The motors here in China are typically 48V or 60V. Most likely your wiper motor will end up being 12V or 24V if it's from a truck or bus. Ideally use a little Arduino controller as your steering brains and just limit maximum PWM to the motor.
Alternatively find a suitable switchmode regulator to drop your bike voltage down to the wiper motor voltage.
Trike by wire! How cool is that?
Or: cables through -erhmm- sheets? Like bike brake cables?
Tadpole recumbent trikes have to deal with a similar situation. You might find some inspiration there.
or another linkage idea
Safety, Please evaluate what will happen if something breaks while you are racing your contraption at "normal" speed. Then multiply with the probability.
Cables, wires, hydraulics, servos etc. are fancy, but perhaps less reliable. Option one, a set of welded bars and linkages should be possible to make damn sturdy!
Consider making the steering in a way that will make you drive straight forward if you take your hands of the steering bar. Dunno what it's called, but it's the opposite of a racing bike.
Final thing, the horizontal bar you have on the image right here, consider moving it further forward, flip the "horns" 180deg, pointing forwards. will give you better loading space for your cargo.