Russ Paden hasn't been around recently. If anyone (Terry Bradshaw aside) would offer an experienced answer, it would be him. He has been refurbing Royalex canoes since the earth cooled.
Personally, I would not paint one. I would fix up any major dings with thickened G-Flex (tinted to match) and call it a day. Anything you paint on the hull, be it Rustoleum, Pettit, whatever is going to fade and scratch back down to the vinyl layer. There a folks that lay on a coat of G-Flex epoxy...the process to do that is simply not worth the effort.
What I do with these old hulls is first give them a bath (Dawn dish detergent and vinegar) and then after the first layer of grime is gone I carefully re-wash using a 3M scouring pad and gently go over it again. I'll then buff it out with a marine wax. I have done this to about a dozen of these, all different colors, red, blue, dark green, disturbing green, banana. They all have ended up looking pretty good. You won't pass them off as new but let's face it, a 1976 hull owes no one a thing.