Gas or electric drive?
I think a gas motor would work best on the rear wheel. If you only drive 1 front wheel it will pull like crazy to one side. The big problem with your typical gas motor is the extra frame tube is right in the way. If you cut that out, I think some extra reinforcements would be needed to strengthen the frame. Maybe some gussets in the corners. The cheapest gas motor kits drive a sprocket bolted to the rear spokes. That's not a very good design. They tend to rip out spokes and the sprocket is hard to get perfectly centered and squared up so the chain runs smooth. A step up in the kits will get you a rear hub that is threaded on both sides so the drive sprocket threads directly to the hub and will run true. No ripping out spokes or throwing the chain. Those use a freewheel on the pedal side. But then you need to add brakes and you will need decent brakes. Adding brakes to the front wheel will be quite exciting. The bike will really pull to one side or the other when you hit the brakes.
I could see using 2 matching electric hub motors on the front wheels. Connected to the same battery pack, and controller, they should get equal torque. That trike design uses pretty "normal" heavy duty front bicycle wheels. No thru axle, differential, etc. Those front electric hub motor wheels would just bolt right in and don't have any extra drive mechanism. Battery packs could be put in the bottom of the cage in/under a seat or cargo box.
rick
