CA put an end to this. There used to be a loophole that allowed "fuel type swaps" that some people at some DMVs were doing.
Now any fuel type swap has to go through the BAR, same as an engine swap. The new engine has to be newer, from the same weight class (no cummins, duramax, powerstrokes, isuzu, which only came in medium-duty or heavy-duty trucks). They have to be mated to the original transmission with all the computer controls in place. The BAR will fail anything they can't look up in their books. Your documentation is irrelevant. If you're planning a swap you should get it pre-approved as much as possible.
A VW, Toyota 2L, Nissan SD-33, 6.2, maybe a 6.5 or mercedes diesel swap is still possible.
CA is not going to allow any imports of anything that was not originally imported into North America. I suspect it still may be possible to import a Canadian diesel that's > 25 years old.
I personally know some people who have imported vehicles that can now not get them registered in CA. I have also met some folks who have stuff like hj75 pickups with legal CA reg.
The bottom line is that there is no way to import a post 1975 rig from anywhere except perhaps Canada and register it in California.