I think that is one cool looking little trailer. Is the box portion stainless steel?
I agree, from the pictures, the undercarriage does not look road-worthy. I would suggest you stop by a couple small welding shops with those pictures in hand, and see what they say. Craigslist might have some ads in the services section for people that do repair work like this. I am guessing a trailer shop is just going to try and sell you a new one, but I could be wrong.
As far as replacement parts, a google search came up with this: http://www.northerntool.com/webapp/w...0531_200330531
Sure, you could buy a new one, but your father and grandfather didn't build one of those!
Post some pics if you do end up rebuilding it.
BTW, where in the USA are you located?