Battled with this for months way back. I don't believe there is a cure for the original. Once the vinyl gets brittle enough to crack, it's done. Patch one place, a month later another pops. Eventually you get more patch than original cover. The most invisible patch is put on the back of the cover. Buts that leaves the crack edges to catch on whatever slides over it. I didn't know about shoo goo back then, which would help with the curling edges. But, it will shortly crack on the opposite side of the bike pretty quickly. And you will chase seam cracks as a way of life.
Best to just get a new cover and reset the vinyl clock, I'm afraid.
Cheers,