I think the shipping costs will be very expensive from Japan to the West Coast to ship your engine, have it rebuilt and then shipped back. I would recommend that you rebuild it yourself. They are not a difficult engine to build and the machine shops in Japan are very precise so you should be able to have the machining done in Japan and assemble it with parts that you have shipped from the United States. Failing that, you might save half the costs and just buy a complete engine from the United States. There are several builders who show up on the Facebook groups that I have seen your posts in.