625 ILCS 5/1-118
All integral and body parts of a vehicle of a type required to be registered hereunder, the removal, alteration or substitution of which would tend to conceal the identity of the vehicle or substantially alter its appearance, model, type or mode of operation. "Essential parts" includes the following: vehicle hulks, shells, chassis, frames, front end assemblies (which may consist of headlight, grill, fenders and hood), front clip (front end assembly with cowl attached), rear clip (which may consist of quarter panels, fenders, floor and top), doors, hatchbacks, fenders, cabs, cab clips, cowls, hoods, trunk lids, deck lids, bed, front bumper, rear bumper, transmissions, seats, engines, and similar parts. "Essential parts" also includes fairings, fuel tanks, and forks of motorcycles. "Essential parts" shall also include stereo radios.
An essential part which does not have affixed to it an identification number as defined in Section 1-129 adopts the identification number of the vehicle to which such part is affixed, installed or mounted.
"Essential parts" does not include an engine, transmission, or a rear axle that is used in a glider kit.
625 ILCS 5/1-118