Whats kind of power does a 600 make, maybe 90hp? You can get a cheetah cub and make 100+hp and it be more beneficial in a lot of ways. For one, it weighs a fraction of what the ninja motor would weigh and because of that, you would retain the same handling characteristics as a stock banshee. As far as cost...I understand the cub motor would cost a bit more, especially if you already have a ninja motor sittin' around but there's nothing quite like the feel of a helacious 2 stroke powerband!!!
I think it would be a cool project to embark on...just hate to see you chop up a perfectly good banshee and get frustrated and quit in the middle of the build like so many others have just to get horsepower when you can go the easy route and upgrade to a cub and get the same affect, maybe even better for a lot less brain damage...
just my .02
I agree though, if you decide to do it, take photos along the way and keep us updated!