either your brake arm is not secured to the frame, or you toasted your hub. They dont often put too much grease in them at the factory, so greasing the entire inside of the hub before operating the motor is a good idea.
I think the brake arm is stripped!
Take it off the hub, & I'll bet the center of the arm is destroyed.
When you bolt the arm back on the hub after intalling a engine sprocket, you have to torque the sh** out of the bolt that holds the arm on! (& you also have to KEEP the bearing dust cover on)!
plus, when it gets worse, it can start pedaling forward suddenly at the speed of the wheel's rotation. like a fixed gear. and that's not something you want to happen at 30+mph.