100% completly aftermarket. As far as I know there is only the L&L designs (formerly L&A) replacement one way for the raptor. They are a great unit built really nice. Ripper racing has a complete how to on it, step by step no messin around. Pretty easy with a couple tools and a little elbow grease.
The oneway bearing is the clutching link between the starter and the crankshaft. When the starter turns the bearings of the oneway bind agienst a gear on the flywheel. when the motor is running the bearings release the gear and the starter stops. The flywheel has a 52 tooth gear on it, it's notorious for failing along with the oneway bearing and should usually be replaced as a set.
A couple of links for more info.
And by the way, the 01 are known for the tranny's failing, ALL raptors have a oneway problem.