Easy Fix
TAKE OFF THE SPARK PLUG WIRE( FOR SAFTEY SO U DONT TURN OVER THE ENGINE AND START THE BIKE)... THEN DRAIN THE TRANS OR OIL IF IT IS A 4 STROKE. THEN TAKE OFF THE FSPROCKET ( one bolt) slide it off and there is the gasket (i believe it is called an output shaft seal) anyways its like $3.50. Use a small screw driver or a pin or anything to pull the thing out( now for putting it together) put some lube such as wd40 of trans oil and coat the gasket. This will make it slide in easier.. put it arround the shaft just enough to hols it there ( make sure it is strait..) Now take a socket about the size of the gasket (1in to 1 3/4in) and put it arround the shaft (this will allow you to push in the gasket equally strait . you can then push in the gasket, you might have to tap the socket with a mallet of hammer to pust in the gasket. now put on the sprocket fill the trans with fluid or oil ( depending on 2 stroke or 4 stroke) and put back on the plug wire and your done start it for 5 min and check for leaks