Good point about the registration cangus. I can try to look up info and see if out of state registration will work for a California bike. I was thinking of giving that a try by selling the bike to a friend in Nevada, get it registered then buy it back slightly more for his trouble.
If that doesn't work, nxrsr20 has a good point of simply bolting in a smaller engine with less headaches then an overseas only sourced engine. If I knew of a proper engine that bolts easily into the NSR frame, i'd have done it years ago. It's never too late to still give that a try.