it would seem logical that the engine swap would cause a compatibility issue with the rear sprocket- engine case covers were different between the 500 and 550, but i have come across folks running this combo in the past, and none has claimed this to be a problem. at least one current member of the board is running this set up and would be able to give you a definitive answer. what you need to do is start a new thread that asks that specific question and you may attract a fellow 500/550 hybrid owner to answer you.
-KK