Sure, it would work fine. After all, they are all 8 ohm speakers. Any 8 ohm speaker will work. The SP-31 will probably sound better than the SP23, since it uses a larger diameter speaker. The only thing "special" about the Kenwood speakers is that they match the color/style of the rigs. Plus, the SP-31 has a couple of switchable, passive filters.
As was already stated, try a left-over stereo speaker, you'll like what you hear.