In most (if not all) cases, Honda Hybrids remain emissions legal,
thanks to the low emissions levels of Honda engines. However to stay
emissions legal, you will have to retain all the emissions hardware
that came with the original chassis/engine or, even better, uprade the
emissions hardware to the one that comes with the new motor. The
essential hardware that is required for legal emissions are: catalytic
converter of the correct type, properly functioning O2 sensor, properly
function evaporative emissions hardware.