Any modern Motorola 8-ohm speaker should work just fine with your scanner. In the past, Bose held the contract for Motorola's amplified and non-amplified speakers, and they sound fantastic. You can find plenty of them on e-Bay for sale. In fact, here is an
example of the non-amplified 15 watt speaker in question.
The older Motorola metal speakers that are black, with a white face, and a red "M" that were designed for the Mocom 70, Motrac, or Motram radios, were manufactured in the 1960s, are rated at 3.2 ohms, at 7.5 watts. In order to use one of these great speakers, you would need to add a 10 watt, 5-ohm resister with the speaker wire, so the radio will see a balanced 8-ohm load. If not, you could over a period of time, damage the volume control on your scanner.
73's
Ron