What about a BionX D with external controller?
Motor weights just 4kg and with a 20s battery should easily reach 50km/h.
I don't know if you would be happy with the maximum torque, I assume 60-80A phase current should be a sensible limit and that's not for continous useage.
You can try the Simulation, the BionX D is included in the motor list: http://www.ebikes.ca/tools/simulator.html
I also don't know which controller works well and which doesn't.