K-head speaks true.
These things are designed as a system. A box is designed to fit certain speakers and unless they are very, very similar, you'll lose performance.
Add to that that many of the amp/crossovers built into these things is designed for the original speakers in that enclosure.
So, yes, you can buy speakers that may fit and it will work, to some degree, and it will change the sound, but it may not be quite what you're hoping for.
But, hey, it's your money. Have fun.