How did you manage to ERASE THE FIRMWARE (moreover : ACCIDENTALLY) ?
In any case if you destroyed ROM maybe "something" is possible (but not what you think i.e. brutally copy another ROM : this is exactly why simply swapping a PCB won't work).
If you erased completely the FW part on the platters, forget data.
An attempt is possible with 60-70% (rebuilding all) but it's gonna cost you a lot of money and there's no implicit guarantee about getting all data back.
Finally I can make the drive work again (rebuild all , will destroy everything on drive) but you have to send me the drive so it's not convenient.