i have a 1979 johndeere trailfire, it used to go like 50 mph, but it has been getting slower, and slower. it sometimes makes a shuttering sound when im trying to get going or going up hills, the belt is good though. it starts and idles good,but its lacking power,could it be crank, clutch, drive? i need some help,please help me out with what you think it could be,thanks for your time.