That's interesting. I've always attributed the entire cover to Kirby/Coletta, but looking closely now, I agree that Marie Severin had at least a hand in the portrait. The eyes, in particular, don't display the typical Kirby shorthand. So now I'm wondering if Stan ordered a correction, if the Severin Thor was pasted over a Kirby Thor. Or, alternatively, if the Thor figures by Kirby are stats taken from elsewhere, perhaps the interior of the book, and used to set off the Severin head. In other words, any new Kirby art on this cover?
I checked that issue and the 3 previous issues and did not find any of those stock Kirby figures. It was a last minute reprint issue so it is probable that they did have a cover that needed to be altered.