1/8 buggys with outrunner power

[DLS II]

Hi, I'm thinking of converting a Kyosho MP-6 to brushless using a outrunner to replace the center diff.( I know it's easier using an inrunner mounted next to the center diff, but this intrigues me) How big an outrunner do I need? I saw pics of someone else's conversion and they were using a HXT 50-55-450. Do I really need one that big or could I use a Rimfire 42-50-1000? Thanks, Don