However, all is not lost and perhaps we can do this with a 50kv motor on 24V and single reduction of around 10:1.
So we now have 50kv x 24V = 1200 rpm. then a 10:1 single reduction with 120 rpm now at the crank. The Vesc has no problems at a 50% duty cycle or even a bit lower so no problems tuning the crank at 50 rpm.
And now the engineering.
So the mount is easy and can be mounted up nice tight in a similar position to my latest bracket but using the ISCG-05 mount.
In regards to the drive system, I would consider using a #25 chain and sprocket combo. This will keep the sprocket diameter quite compact and should be plenty strong enough to handle the power output of this motor combo.
Keeping it simple,the motor would be connected direct drive (via reduction) so the cranks will turn on there own with the drive is activated and when free pedaling, the motor will be driven. Not sure how much drag this will place on bike when pedaling but it may well be quite minimal. However, if the drag is too great, I would consider using a sprag clutch on the motor side. This wont stop the pedals turning under power but would remove the drag on the system when free pedaling.
So it looks like there might be another project in my future.
Best I finish this one first.