If my EK9 had the original radiator when I bought it then they are identical to EK4. It was leaking and I swapped it for EK4 and have used it without any issues whatsoever. If the original EK9 radiator does have bigger pipes then I've learned something new! Never heard that before though. I know D series engines use radiator with smaller diameter pipes.
I've only ever broke b series cars and I have loads of the 28 mm rads but the ek9 I have atm has the 32 mm rad and it says honda so abit stumped, I've used the smaller neck one and made sure the jubilee clips r tight so it's fine