Hi quick, if I may answer.
I dont think its advisable to mod the HU on the new civic bec its built in and will waist the steering audio controls as well as warranty. But if you really want to change the HU then maybe you can look for a place in the dash to place the new aftermarket HU and let someone who really knows the civic to disconnect the HU conector at the back. I just dont know if you can remove the radio itself since its built in.
Now, if you are really rich and is ok to spend, go to I think US civic where in their radio are 2 dins. You can purchase the whole dash board and change it here so that there is a provision of the 2 din radio that fits perfectly. But this is a very wild idea and for those who are mad with money (in my opinion).
However, will suggest that you place an amp to power up the 4 channels (placed at 2ohms) and add a sub at the back. That will be balance enough for listening. If you want more bass, place two or three subs.

As long as its in the budget.
Hope this helps.
God bless!