I'm afraid that Ellison had the delusion that his script was art. TV episode scripts are completely subservient to other needs of the show - actors paraphrase, directors cut for time and pacing, producers change stories if the episode is over budget and there are no more funds for special effects or charming aliens. I thought that particular episode was very pretentious, and I bet that's after Roddenberry ticked Ellison off by adjusting it as needed.
A quick summary of what got changed:
The reason for the entire incident was massively reworked. Instead of McCoy going temporarily insane, it was a never-before heard of crewman who had been selling futuristic narcotics on board the ship. And his reason for jumping in the time portal was an attempt to escape being punished for his crimes.
Wonky part: the original reason for beaming down was that Kirk was going to leave him stranded on an uninhabited planet as a sort of improvised death sentence. Then they realize that there's funky ancient ruins near where they dropped him. One of Ellison's revised drafts changed this to the guy beaming himself down to get off the ship.
Guardians; the basic Guardian of Forever concept was in the original. However, there were FOUR aspects in the original: A; the massive ancient city, B: the time portal, C: the nowhere to be seen unbelievably ancient race who created the portal, and D: guardians who watched over the portal and operated it. Yeah, instead of an incredibly confusing to talk to AI, it had a group of even more confusing living being who had seemingly gone half-mad from living so long they don't even remember how old they are.
Side note: in the original the titular city was not supposed to be 'ruined"(old and dilapidated) but rather "runed"(old and decorated with writing). This was because the guardians live there.... and... who knows who else.(no one actually looks around)
Missing section: in the original, just after the timeline is wrecked, Kirk tries to beam up to the Enterprise and finds himself on board a vessel run by pirates... however the tech used is so similar Kirk was able to remotely activate the transporter to automatically beam him up, then Janice Rand has to hot wire the teleporter to beam Kirk and Spock back down.
WTF plot twist: Edith Keeler's role in the original was in many ways the same, but in some ways completely weird. In both, Kirk gets romantic with her, and in both, it's her non-death that changes the timeline. However, in the original, Kirk and Spock fumble through figuring out what to do based on some weird prophecy the Guardians give them. the televised version has Spock look up Edith in a database and realize she's supposed to be dead. Also, the original had no explanation for why that guy who was a drug dealer and murderer SAVED her. Also... in the original, the timeline restoration was a complete rug-pull. Kirk and Spock at no point had any inkling her death would fix things. Then she dies and Kirk is confused and sad. It didn't even explain it after she died. Instead Kirk sits around pondering the bizarre cruelty of fate.
double WTF: Oh also in the original there was a third use of the portal. This third use was to non-execute the bad guy by throwing him into some sort of infinite death time loop.
So over all, as a stand alone sci-fi story it'd have been great... but as a Star Trek story? not so much.