I recently bought this 85 FLTC which is in really nice condition and seems to have been fairly well maintained but I am concerned about the drive belt which is not damaged but is as old as the bike. I know I wouldn't trust tires that were this old no matter how good the tread was but how is it with belts? Do you just go by mileage as this bike has 21,000 miles on it ( sad huh?) Do they get weak with age or from just sitting? Any thoughts?