I wouldn't be too worried about the power handling rating of the speaker. You need to be more interested with the ratings that determine what the speaker does with the power supplied to it. You need low ohms and high sensitivity. You can tell from the original speakers 5.5 watt max input how little power that box is producing (probably only between 3-5 watts per channel @ 3.2 ohms - and that was when the box was new. That power rating will go down too into a 4 ohm speaker.
You also need to check the frequency range of the speaker to ensure it can operate well in a two way setup with no crossover that removes the highs going to the woofer.
Some drivers are more suited to a three way setup where a mid driver takes up the mid frequencies that some bass drivers can't produce well or at all.
I can't really recommend a modern speaker without knowing the space limitations inside that box. One old school speaker I can recommend are the drivers from a Sanyo M9998K - slightly lower 3 ohms, big chrome caps and that typical bassy Sanyo sound - well suited for a low power application.