You could build up the shaft with tape and press-fit it on.
You could try mounting a small gear on the end of the motor (that press-fits), that the disk can be press-fit on.
You could use 2-part epoxy to fill in the center of the disk, then drill out a hole in the center of the epoxy fill.
You could try to find a larger gear (or pulley) that will press-fit onto the motor shaft, then glue the disk to that (or mount with screws).