NT was designed to be portable, so the machine specific parts are cleanly separated, even on the same CPU. So if you port only those parts, the existing binaries for an architecture should work. The three bits are:
1) ARC boot firmware. NT was developed on non-x86 systems like i860 then MIPS, and ARC is the native boot firmware used (on x86 NTLDR emulated it prior to Vista). Similar to a PC BIOS / UEFI, and a PCI PowerMac would use OpenFirmware. As well as providing an ARC compatible environment that loads over OpenFirmware, this project seems to does some fun so the boot firmware can pretend there's a storage device so that driver "floppies" can be loaded during the initial stages of setup. (ie an F6 disc)
2) A HAL. The main NTOSKRNL is hardware agnostic, so the idea is that there's one binary for each CPU architecture. But the kernel needs to interface with actual timers, busses etc., so the interface code is in HAL.DLL and the appropriate one is copied by setup. (For example see https://www.geoffchappell.com/studies/windows/km/hal/history... for a list of x86-32 ones in older versions of windows, with various HALs for NEC PC-98, IBM MCA, various early multiprocessing systems, nowadays there's just one AMD64 one that's mostly in the kernel itself). So the main kernel in unaltered, and halgoss handles the Mac specific stuff.
3) Device drivers. Once NT is up and running it does need actual drivers.
The specs for 1) are known, so can presumably be emulated, and I guess there's a DDK for 3) (or it can be deduced from another DDK), I guess 2) is probably the one that would need the most insider knowledge, I'm not sure if the leaked NT sources go down to that level as I've never looked at them.
As for compatibility, 32 bit PowerPC Win32 binaries, 16 bit x86 Win16 binaries, and whatever x86 DOS stuff will run in an NT4 DOS box. No x86 Win32, only Alpha had an emulator for Win32 x86 stuff (until ARM stuff).