Hey guys... new as a Valk owner (about 4 weeks now), and new to board...... this makes my first post. I had the EXACT same thing happen to me - I just wasn't lucky enough to have a tow truck driver that could whittle a wooden fitting - but after searching methodically on a Saturday for a radiator repair shop, or anyone that could possibly weld it, we (my father-in-law and I) decided to attempt soldering.
Let me just go ahead and put out there.......... DO NOT TRY THIS AT HOME.
I ended up taking it to a welder that had an arc and tig welder, but he made me use his tools to clean the radiator mouth off where the nipple broke off, as well as the nipple, then welded it back on, about 3/4 of the way around; on the sides and top. He said he
would have welded the underside of the nipple, but did not want to risk getting close to the seal at the top of the radiator.
That said, after his welding, and bit of JBWeld epoxy from the parts store, and I was back in business.
Of course, I went ahead to drop it off at the radiator shop, and have them pressure test it for $25, just to be safe.
All in all, a full weekend of "do it yourself" attempts, a friend-of-a-friend that welds, some epoxy and $25, and I was able to put it back on, and running clean with no drips 
I hate that someone else had the same thing, but I'm glad at the same time, to know it wasn't just me. The bigger question is, how many others have had the same experience?
Oh, and just in case anyone out there is unaware -but I doubt you are- always insure to use a 50/50 with coolant and distilled water. Father-in-law clued me in about the fact that tap water to an aluminum radiator can be as bad as pouring acid on a penny. As well, when I spoke to the welder, considering the small amount of alum in the nipple, there's a good possibility that the water used prior to my purchase was tap water, could have weakened it enough to break off.
So, now that I've written a book as my first post, thanks for the opportunity 
Ride on - ride safe
