Do the belts and straps lose any of their strength after 36 years of wear and weather? Is there a way to test them? Don't want to find out the hard way.
I think it would depend on how much UV exposure, water etc. they have been exposed to...and I'm guessing that the only real way to test them would be destructive.
I'd guess that if they appear to be in good shape then for the most part they would do the job in an accident
Good test is hold the belt upright, if it stands up on it's own like a raging chubby, get new ones. Just my opinion. I'm talkin' about the belts to be clear
One thing to look at, like Duncan said, is UV exposure. Dry fibers are going to be much more brittle than newer ones. If the belts are stiff and dry, throw 'em out!