What should be a simple premise is fraught with complications, the realization of which didn't come for some time.
Dr. Donald Blake is an ordinary surgeon, who presumably was born of human parents and has been alive for some decades. He finds a magic stick that, when knocked on the ground, transforms him into the Norse god Thor.
At first it seems similar to Captain Marvel/Shazam - Blake remains Blake while gaining Thor's powers. Blake has all own memories and personality (no thous and thuses) and at the start doesn't have additional memories - he only knows Loki from his knowledge of mythology.
But what of the Thor of legend? At the first issue we don't know if he existed before, if he's still around, or if Thor was always a series of mortals granted power by Mjölnir. Very quickly, Blake begins to become more Thorish, and addresses Odin directly as if from personal memory, and that the Thor of legend has been missing for some time.
Thor gets powered up pretty quickly. Tapping the stick once changes Blake into Thor, once again changes him back, two times lets him control weather, and three times stops the storms. It's complicated, and I think quickly abandoned, as he can control weather from the skies as well. Blake is even able to control the weather without turning into Thor.
Technically Thor can't fly - he throws his hammer and grabs onto the handle thong. But since he can control where the hammer flies, and can re-throw it a different direction or spin it around and stay in place, it's functionally the same as flying. I will keep my eye out to see if Thor flies or floats when he's not holding his hammer.
Other powers include emitting anti-matter particles from his hammer, blowing hurricane force winds out of his mouth, tracking items by taping fragments to Mjölnir, and travelling through time. He's the Silver Age Flash of Marvel Comics, inventing new powers as needed. I remember these are explicitly removed later, not just forgotten about.
We have our first "could she ever love a cripple" romance comic anxiety piece, and our first of the few Marvel heroes to have a cape
Questions to keep an eye out - When does Thor start speaking in ye olden tongue? Is Thor bulletproof? My memory from my first read is that the handbook entries say all Asgardians are denser and bullet resistant up to 50 caliber, but that he's never actually been shot.
Journey Into Mystery 83-87
Collected in Thor Epic Collection: The God Of Thunder in paperback and Kindle.