Do you have pix? I would custom-make a frame to support a Lovejoy-style coupler, if there is room. There are also off-the-shelf frames, but they will need modification to fit your engine, most likely.
Hey, just wanted to tell you and anyone else seeing this that I found a perfect fit doughnut coupler in Korea, and was able to buy the part for $100,less then a third of the Kubota list price, and I had it in hand in 1 day, and it bolted right onto the old flanges and fit perfectly, and the old KH1 is back together and still digging!
Hereç—´ a pic.