DuroMax DuroStar Champion 500cc 20hp Motor

Hi guys.
This is my first post, please bear with me.
I've bought and repaired a few generators that I usually turn around and sell, but a DuroMax XP13000E has me chasing my tail.
It's basically a Honda Chinese clone GX390, but they modified the block and head to accommodate the huge displacement. The motor has a "500 2GV" engine family on the sticker and I noticed the valve cover had 4 bolts instead of the typical one central bolt. I thought I had lucked out when I found a bent push-rod, but the aluminum webbing under the rocker-arm was broken.
When trying to find a new replacement head, I discovered that Champion and DuroStar both had the same motor in their largest single-cylinder generators, even the same part number in their manuals (part number 12140-Z0D0210-0BA9.) I've searched Ebay and the normal generator websites, but except for ordering this directly (big $$!) from Champion or DuroMax, there aren't any to be found.
Does anyone know where this odd-duck could be found? I'm trying to flip this thing without losing my keister!
 
Thanks in advance,
-B