A lot can vary from radio to radio, so these won't be exact steps for yours, but a general method for reusing the case, dial, knob, and speaker when possible.

Disassembly

Most vintage radios are accessed by removing a back panel. Go ahead and unscrew and remove yours.

Next, carefully pull of all knobs. These are often press-fit into place and can be removed with moderate force. Some may have a small set screw that must be loosened first.

Check the bottom for fasteners -- in this case small hex bolts -- are remove them.

The speaker will be connected to the radio electronics by two wires. Cut those right where they enter the board, leaving the full length of the wires connected to the speaker.

You can now remove the electronics from the radio. Look at those beautiful vacuum tubes.

Tuning Mechanism

You'll find a large wheel connected to the tuning knob shaft via a tensioned tuning cord. Try out the mechanism -- if you find that it is slipping you can apply some rosin to the cord, as the whole system relies on friction to turn the cord, which in turn moves the dial indicator.

Here we can see the tuning indicator that slides back and forth. It is to this that we'll be attaching the magnet later.

On/Off Reuse

Looking at the underside of the radio, you'll see a bunch of crusty old parts. Do not lick them.

If you want to have your Haunted Radio turn on and off via the original knob --this is often a dual-use on/off plus volume knob --  check for a pair of wires running to the knob. These are the on/off switches, vs. the set of three lugs that are for volume. You can clip all of these wires and then test continuity with a multimeter to confirm.

Solder two wires to the on/off lugs, about 8" each.

Soldering on your stranded wire is fun!

Here I've removed one of the vacuum tubes to use as a convenient through hole for running wires up to the top.

Battery Prep

Next, you'll splice the on/off switch wires into the battery cable. Start by cutting the black wire (or the red, the decision is yours!)

Strip all of the wire ends.

Use two butt splices to connect two pairs of green and black wires.

Squeeze down the connectors with a pair of pliers and you've now added the radio's on/off switch to your battery!