Does yours have a gas cap that locks? If the gas cap has a tumbler with a code,usually 3 digits and a number,you can go to any lock smith and they can cut you a key off that code.I just did that with a tank I got at a swap meet.The ignition switch may have some sort of code on it ,too.Not sure.There may even be a code on the seat lock itself. Those old codes make life easy for guys like us,but it also makes life easy for guys who like to steal stuff.All you have to do is walk by a guys bike,get the code off the gas cap and come back tomorrow and ride off with his bike
My Pops lost a CB750Four like that once.