The trouble with these custom axles is that the bearings are light duty and the central sprocket is secured by a hollow spring steel drive pin. With normal use these pins crack collapse and break, so adding an engine into the mix is only going to hasten their demise. And once the pin breaks all drive is lost, which is not good if it happens in the middle of a busy intersection.
With good bearings and an improved securing pin on the sprocket the axle might be fine, but if this trike is going to be for an 80 year old I would be concerned about using this axle unless your customer is skilled at changing drive pins on the side of the road at a moment's notice.
