You can get a new lock cylinder / key cut to order from MB that will work with your other doors by using your VIN. The flip key alone is ~ $ 150 , not sure what the lock cyl would be. Near as I can tell , all custom stuff comes from New Jersey so allow for build and shipping.

The flip key / fob has the RFID chip in the rectangular portion. If you installed used parts you would use the metal portion of the replacement key ( so it turns the lock ) and the black plastic / fob that contains the RFID chip so the anti theft releases. If your black plastic / fob is falling apart, the RFID chip can be plucked out and placed in the other housing. You would still need a old key for the door / trunk.

You can get a regularish looking "flat" key with a built in RFID chip that will turn the lock and run the car from MB for about $ 35. The head is short making it more difficult to use. The round "valet" key isn't sold by MB for some reason.