Better question is why did it break? The driveline bearing on 90's Arctic cat sleds is notorious for failing, and when it starts to fail, it drops the driveline enough that the square key that drives the speedo shears in two pieces... make sure you check the bearing when you are looking at it BEFORE the failed bearing causes you MUCHO grief. The bearing is sealed both sides, the grease you shoot into the driveline gear NEVER gets to the bearing. Smart huh.
you take a drill bit the same size or a tad smaller than the adapter that goes in there and just drillit out. The adapter is much softer than the shaft so it will drill easily, the just pound the new piece right in there. good luck!