@ 23W two KM10 will fit side by side, but they will be close. @ 23 x 20 10, you internal volume couldnt be more ideal.

Make sure you get the higher quality birch. Its usually void-free as compared to the lower grade used for subfloor construction. No fiber glassing, just a quality 2-part marine waterproofing.

If your current woofer is a 4 ohm, which I would expect if it was the OEM woofer in 2011, then you would get a new 4 ohm and they would be wired in parallel and then connected to the existing speaker wire.

Your original woofer likely has a silver grill. The new ones will come with choice of charcoal, white or chrome and they look different. If you ask around, you might find a kicker dealer thats got spare KM-10 grills so you could match the old to the new. Just note, the cone of the woofers look a little different. Once the install is done, I dont think the difference will be noticeable.