Just to be clear, does the impedance mean the ohm's must be the same as original? The original are 4" 8 ohm's 3W speakers x2 in the cabinet.
If the new ones are 4" and 8ohm's does it matter about the Watts? Can I go for more Watts or would I then need to make modifications to the machine to accommodate this?
You can go for a higher wattage speaker no problem 
As for the impedance, again you can go higher with only a slight loss in power, which may not even be noticeable. NEVER go lower, eg 4 ohm instead of 8. It can cause an amplifier to short out due to the amp trying to output more power than its capable of, especially if its turned up loud. The speaker is sometimes destroyed too if this happens, due to DC voltage across it when the amp dies, usually you hear a loud hum followed by silence!
funny that, my wife must have a different impedance to me when I get home from the footy late. I get the same result, first I hear a lot of humming or was that yelling followed by a lot of silence.....sometimes I hear a large ringing noise from my head too
