Where to find replacement 5th wheel pin box?

I'm considering buying a 1985 Holiday Rambler 26RB fifth wheel that's available locally from a private party. The unit is immaculate...never been rained on ever, never been cooked in, very little use, etc. It really is in pristine condition.

However, I have a bear of a problem concerning the king pin box. For about 2-3 years (I garnered this info from the HR factory) Holiday made the fifth wheel setup exactly bass-ackwards: the kingpin mounted in the truck bed; the hitch mounted to the king pin receiver on the trailer. This 5th wheel falls into one of those years...weird, huh!!

The owner (understandably!) had the trailer converted to a gooseneck when he first purchased it. I wish to use my conventional fifth wheel hitch, (gooseneck is not an option for me) but I cannot locate the king pin box required to replace the gooseneck, which I need because the gooseneck was welded, not bolted, directly onto the trailer's existing king pin box.

The "receiver" (is "receiver" the correct terminology??) on the trailer measures 12in. square i.d. I have searched every trailer dealer in my area for a king pin box this size, and have been unable to locate one; I found several sizes, but nothing I can use on the Holiday. I've also Googled the internet to death to no avail.

Is my only option to have a king pin box fabricated? And if I go that route, where do I locate a replacement king pin?

Any help on this dilemma would be welcomed! I've spent a month searching for the 12 x 12 box, but no luck.

Thanks for the bandwidth.