My HOTD2 upright has a uniquely cased ROM (grey metal, not white plastic) board running BIOS 21330, not any of the standard releases. It has no Naomi splash screen, and simply says 'Now booting: HOTD2' or some such message in the standard test-screen font.
I can play confidential mission on it by hooking it up to a 21576G BIOS GD-Rom rig, so the rest of the hardware is compatible. I actually leave the 21576G BIOS GD-Rom setup outside the cabinet and use power, audio, and USB extension cords.
However, my HOTD ROM board, when placed in a standard, white Naomi mobo running a 21576 or 21577 BIOS, does not boot, it says the cartridge is unacceptable.
If you find a ROM, you may need a different BIOS as well -- if you are looking for the 21330 BIOS, I'm sure we can arrange to dump a copy. I do not know if there are any differences between the HOTD2 metal-cased mobo other than the BIOS, because I have never swapped the 21330 BIOS out onto a modern white-cased mobo.
I have a Death Crimson OX board as well, and it is uncased as the HOTD2 ROM is, but I haven't been able to test it yet.
If you get board with your MOTK boardset, I would gladly trade you some other naomi soft or PCB, feel free to let me know.
I have access to another Death Crimson OX boardset potentially if you are interested.
I can post pictures if my description is hard to follow.
-Dan