Is it worth rebuilding an old Japanese engine?

For iconic chassis like R34 GT-R, Supra MK4, or NSX, absolutely yes — a proper rebuild can add 50-100+ years of reliable life. The rebuild cost ($3,000-8,000 depending on engine) is much less than replacing a complete engine ($10,000-30,000+). Factor in the chassis condition, parts availability, and your ownership timeline before deciding.