You have a few options...
1. You can buy another set of stock speakers.
2. You can buy an aftermarket set of speakers and a set of spacing rings to space them out properly from the door; effectively ditching the stock "casing."
3. In my opinion the best method - buy an aftermarket set of speakers and pull out your stocks. Look for small clips on the outside of the stock speakers and pull them apart. You'll remove the cone during this process as well, pretty much damaging the stock speaker beyond use. However, when you remove the stock speaker from the "casing" it becomes a perfect mounting spot for your new speakers.
And, yes, you'll have to cut the stock plugs.