The 650 is a slightly different engine as it has a longer CVT unit, plus it is a twin cylinder, although we haven't fitted a 650 yet im sure it could be done, especially with the clever chassis design which should make mounting many engines an option! But we would need a donor engine to try in the frame and make the necessary adjustments,,, but as I say most things can be achieved,,
Can I ask why you are looking for the 650cc over the 400cc ?? The AN400cc 1999 model we have in our demo has been round the world a few times and is still performing well, they are a very robust engine and it still pushing the Eco-Exo-R 90mph plus but then I bottled it!!!