Trikes have a tendency to tip over unlike a two wheel bicycle. Is there any way to try one of these before you convert it? Try going fast down a hill and see if 20mph seems comfortable and safe on that trike. At lower speeds, swerve around to check stability. If you could, I would suggest you try to keep the self-powered part of the trike useable for two reasons. One, it may not meet the electric bike regulations if it is only electric and electric isn't always perfectly reliable and it is nice to know you could get somewhere if the battery dies or the controller burns out.
It may work to put an electric hub motor on one of the rear wheels and keep the handcycle part useable. If you drive a truck I assume you could rig up a foot throttle for the electric motor. Do you have hills or is it quite flat? You can't easily put a larger hub motor on the front wheel because it can break the fork but the back would be fine and then you would have two wheels being powered if you came across mud or something.