You can build a mounting ring out of MDF. I doubt you'll have much issue doing it that way. You can look on youtube for a video on how to make a mounting ring, you can also make them from like HDPE cutting boards and what not if you don't like MDF.
Also the existing amp shouldn't be attached to the speakers, as far as I know lol. Pretty sure the stock head unit is self powered unless there is a premium option but usually in those the amps are mounted in the back somewhere. Either way it should work fine.
Also your link points to some 6.5" speakers.