I agree with FireUp. Any decent radiator shop can probably fix the radiator unless the core is really shot. I recently dropped the Onan genset on my previous coach for serious PM and sent the small radiator to a shop for a good cleaning, test for leaks. They charged me about $75.
If your ESP will cover the cost, I'd get the new one from Onan.
Also, if it were my 14 year old generator, I'd have them replace the drive belt/s, hoses, T-stat and temp sensor while they have it apart. Would be a shame to replace the rad, button it up, then have that pesky temp sensor fail 3 weeks later :S