it seems to me that the 316 / 318 is the most common platform for engine swaps, and also have the most options available.
If you're dead set on repowering an old tractor, you might be better off finding a 318 that needs an engine.
But, I'm sure it could be done in a 317 as well, just perhaps not as easily.
Well I have a 317 (which already has a magnum 18) and a 318 with what we think is a stiff motor. The 318 is my dad's but he hasn't looked at it in almost 8 months. I think the 318 will get a motor swap if the B43G is actually stiff.
I don't have any pics but my dad did a swap in a 317 many years ago. Like late 80s or early 90s. I want to say he used an Onan engine for his swap and there was some cutting of the frame that was needed.
There is a local guy not far from my office that does lots of repowers. I want to say he normally uses Vangard Engines. Not sure how dead set you are on Kawasaki.