What is the law for semi-motor bikes in Illinois/how does Illinois define a "Motor Vehicle""?

If my research is correct, it is illegal to drive a homemade electric mini motorcycle on the street and sidewalks of Illinois without proper registration and insurance (which would be pretty hard to obtain). And as for as I know it IS legal to ride a bicycle on the street and sidewalk. So how would a homemade bicycle with an electric motor that only helped with the pedaling fair on the streets? I believe that in other states it is legal as long as the engine isn't the only thing moving the bike. Would this be alright on the streets or would my creation be impounded?