Most of the assembly process is quite straightforward. The tricky part is to attach the cut-off disk to the motor. Here's how I did it.

Take some ABS filament for 3d printers.

With a butane torch, heat up the tip of motor's shaft, and put some ABS blob (the shaft must be hot enough to melt ABS, otherwise it won't stick).

After putting an initial blob of plastic, power up the motor. Now, heat the blob and the filament with the torch, and start pushing the filament into the blob, keeping it aligned with the axis of rotation. The blob will start to grow.

After getting the blob to a reasonable size, let it cool and shape it up with something sharp, using the motor as a tiny lathe, to obtain a shape of revolution. Add some more filament if the result is too small.

With a knife, again using the motor as a lathe, make the front surface as flat as possible.

Stop the motor, and attach the disk with some superglue. Don't put much - superglue can dissolve ABS while liquid.

Turn on the motor again, and balance the disk by attempting to cut some diamond-containing stuff (tungsten carbide may also do the job).

That's it!