Garage door hangers are well understood and surprisingly simple. Punched steel hung from the rafters.
The hangers carry barely any load. The lifting force is transferred through the chain/cable/belt guide to the header above the garage door.
A 20 year old Craftsman GDO is almost certainly a Chamberlin chain drive model. A replacement is likely a belt drive. They have a slightly different mounting scheme, so you can't just replace the drive unit and loop around the existing chain. But it is pretty easy to substitute the new unit.
First take out the pin that connects the door bar to the carriage. Move to the motor unit, disconnect the wires, unbolt the motor unit from the hanger, and lower it to the floor. It will pivot on the pin at the garage door. Take out that pin. Assemble the new unit, and reverse the process.