Don't just replace those gears; check the front end bearings for damage. Check the shafts for possible fore-aft movement. With luck, the damage will be limited to the gear case (pistons, crank, camshafts, and valves OK). Can't tell the cause but it looks like a gear may have moved (forward or aft) such that all the stress was on the back part of the teeth, breaking off parts of teeth.