Supposing you could adapt the ROM/EFI/SMC to work with the new motherboard. (I'm not claiming practicality here, just possibility), perhaps desoldering & resoldering the respective chips onto a new daughterboard or even a PCI card.

Does that make it a Mac still? You could literally replace almost every component in a Mac except for the motherboard and have it still be a Mac, but shouldn't the converse be true as well? If I used the original case, hard drive/SSD, video card, keyboard & mouse, even the CPU, but replaced just the logic board (swapping the ROM/EFI/SMC somehow), it seems to me it would still be considered a Mac. Particularly if in the process the original logic board was rendered inoperative to head off any claims of duplicate license use.