and yes, this isn't exactly legal... but so what? Neither is driving 60 in a 55 but we all do that.
MY FRIEND WITH THE BIG MOTOR, please do your best to settle down when you go to court. You're right, they're prosecuting you for the wrong offense, but they could easily prosecute you for the right offense (unlicensed, homemade motorcycle) and they could give you a disorderly to boot, just for fun, if they don't like you. It's your word against a policeman's, a person who is hired just for the purpose of giving fines to roughnecks and weirdoes like you and me.
Before seeing the judge, get a good haircut, cut off any facial hair (it grows back easier than a $500 fine). Get a light colored shirt with a collar... something you'd see on a golf course or in the office. If you look like a nerd, you're doing it right. Cover up tats if you got 'em. This isn't about what's right, it's about what will win it for you.
Explain it to the nice man or woman the way you've explained it to us (this is trying to save gas, using the tools you have, gets 100 mpg, you didn't think it was under any jurisdiction or breaking any rules, and you only turn on the motor when you're in the middle of nowhere, with no one around) Don't talk about how wrong it was to be arrested, how you were mistreated and not read your rights and abused and treated like ****, these people just don't care. This will all ride on you having a pleasant personality (or faking it, dude!)
If you can bring some pictures/drawings and get them excited about your contraption, the way we're excited, that could help you.
Remember, we can't read about your fun experiments with dollies and huge motors if you are in jail or if you are broke from paying huge fines. That would make all of us really sad, because deep inside we are all dreaming of riding your homemade contraption.