Best speakers I know of that will fit are the Polk Audio DB401s
[ame=http://www.amazon.ca/Polk-Audio-4-Inch-Coaxial-Speakers/dp/B000P0R6GQ]Polk Audio DB401 4-Inch Coaxial Speakers (Pair, Silver): Amazon.ca: Electronics@@AMEPARAM@@http://ecx.images-amazon.com/images/I/41Cgy6jdaAL.@@AMEPARAM@@41Cgy6jdaAL[/ame]
Hopefully someone will come up with an AMP that is small and powerful and will hide somewhere on the bike without sacrificing useful storage space.
The speakers are designed for marine use and as such they are weather resistant AND for a 4" speaker they sound pretty good. With an amp the actually sound VERY good