Are you designing an actual radiator, or specifying one to be purchased?
The radiator manufacturers I deal with (in the UK) have full details of the water cooling requirements of all the major engines (MTU, Cummins, CAT, Deutz, Perkins etc). I represent one of these, and get regular calls from radiator manufacturers, to check that their data is up to date.
Generally, the radiator manufacturer needs:
The maximum ambient temperature (air on)
Glycol % (for low temperature areas - perhaps not for some places, say parts of africa where temperature never falls below 5C or so) - but remember you still need to add an inhibitor to the water to prevent engine corrosion.
Noise level
Frequency (for electric driven fans)
Engine rating / speed.
Whether the engine has an engine driven fan - this determins the airflow available.
Charge air cooler - air / air or air / water
As speco says, a margin has to be added for design - diesel engine radiator kW duties are usually given gross by the engine manufacturer(i.e. with engine heat tolerance included), but not always.
A margin has to be added for radiator fouling, both air side AND water side. This is, in my experience, where you have to watch it, since this is where the unwary can be lead astray!