I wouldn't be overly concerned about impedance, although it wouldn't be hard to match. Measure the DC resistance of the existing speaker with an ohmmeter. Advertised impedance will be about 25% higher. If the voice coil measures 6Ω or so, it's an 8Ω speaker. If the voice coil is around 12Ω, it's a 16Ω driver.Do find a speaker that will handle the power though. That shouldn't be hard either, because 20 watts isn't a lot of power. But sometimes guitar speakers don't handle a lot either. So make sure the speaker you choose will handle the power.