The Spectrum internal speaker is not a robust manufactured one. The two tiny wires that come from the inside are directly soldered to the outer pads, that in turn are soldered to the pins that go to the PCB. This means that a rework on this pads may lead to wire brokerage and thus, rendering the speaker useless.

An identical replacement exists. It's present in some kind of cheap earphones. The problem is that nowdays, earphones have become very tiny, displacing earlier models. In fact, the last time I have seen such a earphone with an identical microspeaker was the earphone that came with my DAP Jukebox Player.

So, a modern replacement is needed. After some digging, I've found one that fits the place: it's Digi-key part 102-1557-ND. You can see it here (right), alongside an original Spectrum speaker (left):

It's a dynamic (not piezoelectric) microspeaker, with 32 ohm impedance, and 200mW power.

As you can see, the new microspeaker fits perfectly in the space provided on the PCB.

There's no problem with the tracks passing close to the speaker chassis, because the plastic speaker cone covers the chassis metal at its edge.

After a drop of SuperGlue(TM) and soldering the two wires to the pads at the PCB (no matter which wrire connects to which pad), you're done.

About how it sounds? It sounds much louder than the original microspeaker. So, maybe it's not recommended for past-midnight Speccy adventures on your livingroom while you family is sleeping. On the other hand, a speaker like this will make a sound amplifier to be unnecessary. Here you can hear it, playing one of the tunes from Gyroscope: