14 foot pounds (19 Nm) on the oil tank drain plug. Sounds like it's time to remove the clutch cover and check the belt and clutches out.Top speed should be at least 55+ May just need to blow out all the belt dust from the clutches(in an open area) and fit a new belt if it's glazed or worn. If that doesn't help that much,then probably time to replace the fiber washer and brass washer on the drive clutch along with the 6 guide buttons. Rear clutch has 3 rollers that you need to check out. Plus the drive clutch can be removed and let a dealer rebuild it if you don't want to get that involved.Rear clutch you should be able to rebuild if necessary.If you don't have the manual here's one that goes into detail on clutch repair.2003 Polaris Sportsman 500 Service Manual PDF Download
Plus several videos on this.