would an updated A-1 skyraider make a good Close air support / coin aicraft?

Perhaps. Air Tractor is making the AT802U which is comparable in performance and payload, but doesn't have any takers yet. There are lots of COIN aircraft out there doing just fine.... and most air strikes are carried out by attack jets operating at high speeds and medium altitude out of range of small arms fire. Add to that attack helicopters and drones, and there is little use anymore for a slow, heavily armed COIN aircraft.

No, not really. For one, it's a relatively big, relatively slow aircraft, meaning it would be a relatively big, relatively slow target. Second is that the visibility wasn't nearly as good as with the modern cockpit-forward designs. Third is that it would be expensive to build and operate compared to your two examples. Don't get me wrong, it was a great aircraft, but for the modern world it would essentially have to be an entirely new aircraft from spinner yo tsilcone. The Republic A-10 has been the best close support aircraft designed to date in the USA and an A-1 couldn't really compare or compete with it. Now, the A-10 isn't really a COIN aircraft nor would a Skyraider be one. They're not small or light enough to fall under the definition of one. Nostalgia is great, but lets leave bygones be bygones, shall we?

And we have the C-130 Gunship and HH-60s/Blackhawks.