We had an old Makita cordless circular saw. The battery died long ago and new batteries cost more than a new saw. It also happens to be geared nicely.
I sawed off (I recommend a second saw for this) the saw's handle and battery mount. I also removed the on/off trigger and mounted it on the handle bars.
To engage the power wheel, I used a hole cutter on a drill press to make a nice little disk with a center hole. I mounted this little wheel on the saw blade spindle.
Without damaging the trike, there are three good bungee cord mounts. The two plastic axle covers and the rear screw hole for the seat. The trick is to get the bungee cords to hold the motor securely and keep the friction wheel pressed to the tire.