You can go higher on the RMS, it won't hurt anything.
Did the factory speakers say what ohm load they are?? Most factory speakers are a higher than average ohm load, like around 8 or 10 or 12 ohms. Almost EVERY aftermarket speaker will be 4 ohms. That means that the amps would push a lot more power when turned to the same volume level as before and if they aren't built for it, then they wouldn't last, the amps I mean.
I'd bypass the factory amps and either run new speakers on just the deck, or put them on a new, nicer amp. Also ,depending on the size of the speakers you are replacing, it's going to be difficult to find speakers in the 20 watt RMS range. Most will be 40 RMS or higher, but that is not a problem. Don't try to look for speakers that only take 20 watts. Also PEAK wattage means next to nothing. Only comes into play if you were putting a 100 watt peak speaker on a 500 watt peak amp. Not likely you'll do that.