Hello rh hutchins,
Sounds like pivot parts stiffened up from sitting and they need to be exercised & lubed carefully (WD-40) to get them working properly again. The rubber on the friction disc looks to be in good shape and not worn down too bad. Don't get any lube on the rubber or metal drive disc.
Make sure the main drive belt and tensioner is good and lube the pivot points of the rubber disc hexagon drive shaft (on the ends) before you go digging into this thing. The drive belt is the most common problem that I have seen.
Parts Tree should have whatever you might need.
Also, these don't work well if the metal drive plate and belt get wet.