Well im sure its no easy task to get a 2 wheeler in. But there's a way to it. Start off with a motorcycle that accepts side car which would make it an offorad motorcycle with side car (3 wheels) Personally I'd suggest the BMW R75 (but you can call it anything that makes sense) It's got a side car and it's offroad. It's low rider two.
How would you have a good looking motorbike without a person inside? In RCR's words: Autonomous cars might be a thing, but a rider is part of a bike's silhuette. You cant have a bike without a rider.
I recently came across a mod that fixes that. Adds a "dummy" or a "race driver" to most in game cars. it can do the same for the bike https://www.beamng.com/resources/driver-passengers-and-loads.2906/