I don't know anything about 450's but check the voltage at your starter solenoid and make sure you have enough current getting to it because that's a fairly common cause of starter malfunction - not enough power to shoot the starter sprocket out to engage the ring gear.
Also, bevsure and check (I had a similiar problem and guys on this site suggested this) your ground strap and clean it and make sure you have a really good ground.
Harvey