[jambo]

I've not done this. However, the motors are going to be physically identical in terms of fitting into the frame.

Your headaches will be to do with electrics. I believe the K9 has duel spark heads as you've pointed out so you might need the coils from the K9. They also have a sensor in the exhaust which I'm not sure the K6 had so you'll either need to move all the electrics across from the K9, and the extra sensors or move only what you need to run the extra plugs. I doubt it's going to be too hard though.

Jambo