Phil,
Well Sir, first off, it sure is nice that you, right off the bat, listed what kind of rig you have and, what the radiator is out of. Many just would say, "I need a radiator, where can I get one"? And we'd all be needing more info to possibly help.
It's too bad they told you that. Unless it was seriously damaged or, corroded, about 99% of the copper/brass radiators out there, no matter what their in, can be repaired. That's not to say yours should have or could have been repaired. I've had half my 7.5K Onan apart, twice for various repairs and, getting that radiator out is actually fairly simple. But, two things here.
One, if one is not mechanically inclined, or two, if your generator is not on a sliding tray, it can really put a hex on the time for repair and, of course, the cost. Those radiators, are actually quite small. When I had mine out the first time, it was not having any heating issues (although the bad temp sensor was telling me it was) and, I zipped it down to the local radiator shop for a quick analyze to see if it was in fact, in as good of shape as I thought it was.
They of course, wanted to "rod" it out so, I figured, what the heck, it's out, and, it would be just that much less on my mind in terms of maintenance issues to have completed. So, I let them do it. It was about $85.00 to do it.
I don't know your situation but, a qualified radiator shop would and could have that unit rebuilt, with a guaranty, in no time. Most likely it would take about a day. But, it seems you're at the mercy of your repair facility. I'd maybe talk with them about getting that old one to a radiator shop for analyzing for a rebuild job. Good luck.
Scott
Scott and Karla SDFD RETIRED2004 Itasca Horizon, 36GD Slate Blue 330 CAT
2011 GMC Sierra 1500 Ext Cab 4x4 Toad
2008 Caliente Red LVL II GL 1800 Goldwing KI60ND