As long as the speaker's power rating is > 15W (which is the amp's maximum power) you don't need to worry too much. The main question is if it's really worth to replace the built-in speaker, which - I heard - is actually not bad at all. The only problem is its relatively small size, but due to the combo's size it will be hard to replace it with a much bigger speaker (maybe 10" is feasible, 12" very likely not).
Instead of changing the original speaker, I would recommend to use an external cabinet, which will give you a much bigger sound than you could ever achieve with any other speaker built into the combo. Use a 1x12" or a 2x12" cab and you'll be surprised how this changes your sound. You could take your amp to a store to try this.