Making Windows NT 64-bit is very different from porting it to a 64-bit CPU. Case in point, the NT 4 (and Win2K) releases for the Alpha CPU were technically 64-bit (so far as the CPU lacked a "32-bit mode") but were functionally 32-bit, with all pointers truncated to 32-bits.

Further case in point -- the AXP64 port (64-bit Alpha) of NT didn't have the ability to run 32-bit Alpha software. If you want that, you have to develop WoW64. So in this hypothetical "port Win2k to 64-bit processors", you would need to create WoW64 from scratch. Alternatively, rebuild a lot of software which is old enough to run on Win2k but also aware of 64-bits.

Or take the AXP approach and literally treat AMD64 as a strange 32-bit CPU, not unlike the x32 port for Linux. At that point you have no binary ABI compatibility with any Windows port, and will need a custom compiler port, and compile all executables from scratch.