Thanks DMace for the photos.
Sorry to stir up the pot over whether the cracks are a problem or not -- I was away for a few days and was amazed to find how long this thread had gotten! I'm a mechanical engineer by training, and I think there's no question the rectangular torque tube with access hole is, shall we say, a sub-optimal design for a torque member. But my thinking is that if Kioti is willing to step up and provide a reasonable repair for free, even if the fix isn't perfect, then who cares whether the cracks are a problem or not? At that price I'm happy to get it installed just for peace of mind.
One of the things you sacrifice when buying anything used is that, in exchange for paying less, you don't get the design improvements the manufacturer has made since yours was built. If Kioti will provide me a reasonable facsimile of one of those improvements for free on my used tractor, that's just icing on the cake. Smart move on their part -- with a phone call from a customer service rep and a couple hundred bucks' worth of kit, they've managed to make me pretty much dismiss this as something that's not worth worrying about.