I am not saying your situation will be the same, but mine clanked for a few years will no issue at all, and I drove it pretty hard. When I rebuilt the motor, I replaced it. Its not a terribly hard job. You have to take the head cover off, and the primary clutch, possibly the oil pump if I remember correctly. Been a few years since I did it, and have done a lot since then.
84'200s (Dad bought new in 84, long gone)
84'YTM200 (sold 02)
85'KLT160 (sold 05)
84' Big Red (Cobra exhaust,good shape) sold in 08
84' Big Red (Parts Bike, runs good though)sold in 08
85' 250SX (New plastic, Clark Plastic Gas Tank, Web Cam Camshaft, Cobra Exhaust, Moose Jet Kit, etc..) sold in 2010 and highly regret!!!
04 Polaris Sportsman (Swapped 400 motor with a 500, Polaris wheels w/Kendra Bearclaws, Moose Fender Flares, Brushguard, Warn Winch)
84 200s (Needs work!)