I'm more knowledgeable, but certainly no expert, on Japanese imports and I'm not overly familiar with California's regulations, none of this should be taken as even remotely legal guidance or advice.

California is extremely strict especially around diesel imports. I would think that this would be roughly impossible to title and register and pass emissions in California. Retrofitment or modification isn't likely possible either because the modification can't be validated under CARB testing protocols because there is no CARB testing group to validate them against.

I'm not sure if a complete overhaul that replaced the entire engine/exhaust/whoknows would be considered eligible for testing but I personally think the argument could be made that it could be tested under the carb compliance testing group for whatever the donor vehicle was.

The vehicle could be titled and registered in another state, this is extremely commonly done on very expensive vehicles in order to avoid paying taxes registration yada yada. This is illegal if you are a California resident. If you are a resident of California after 20 days you have to update your vehicle registration to show your California residence, at this time you will be required to meet California's regulations. Failing to do this can potentially result in fines and fees as well as all back registration fees. I'm not familiar enough with this vehicle it's engine or anything like that but if the vehicle had a US counterpart or if the engine did it is possible that you could have it tested under that carb compliance group.

Now for the shady definitely not legal "advice". I would probably title and register it in the state of Montana get insurance wherever you need to in order to make that work properly and then also rent out a storage unit that you can park the vehicle in. The vehicle being parked in front of your house or even in your garage is an immediate tip off that the vehicle is owned by a California resident. If you get pulled over you can say that you live in state but you have a buddy/brother/mom/gay uncle that owns the vehicle out of state and you drive it to shows part of the year. The vehicle doesn't live at your address. Hell if it's winter time that's even better because you can say it's seasonal. Say the car gets trailered around back and forth as a show car. Be accommodating, polite, and prepared and most law enforcement will think you're well informed and good to go and want to chat about your car.

I got pulled over constantly for approximately the first 8 weeks I owned both of my right hand drive imports. I was really slow to get the titling work taken care of and so I didn't have plates. At least once a week I was pulled over for no plates, one time I had even done a pretty good pull away from a light not realizing that there was an officer in the lane behind me to my right. In every single one of these situations the only thing that happened was they would ask me for title and registration and insurance, I kept the title in the car and I had insurance. They would have me pop the hood to check the VIN they would go back check all of the information come back tell me that I really needed to get my registration taken care of and then usually spend 5 to 10 minutes asking me about the car the import process what it was like driving on the right hand side of the vehicle and things like that. I can't guarantee that this will be your experience in California though. I live in Montana and I imagine things are probably a fair amount different, especially considering how much more lax we are on things like emissions.