I ran the Savage Tide AP as a 4e adventure (it was written for 3.5). From memory, there were a few adventures set on the Isle of Dread (Dungeon 142-145?), starting from a rough landing, to a full-on expedition into the central plateau and into the depths of the earth therein to fight Kopru etc. It was basically set in a 'future state', compared to the original X1 adventure (I've got a copy from the '80s), although you could easily strip out the 'Savage Tide Pirates' bits, and just use the interior adventure sites. About the only part it didn't cover much, was the far north of the island; I added Dragon Turtle in for that bit, but it mostly never got used.
Certainly the relevant bits of the Savage Tide AP are a good way of fleshing out parts of X1; the whole adventure path was pretty cool, and from memory the Isle of Dread pieces were very good. In fact, from my distant memory, the back-plot of the Pirates was also very good and even if you don't play Savage Tide in full, it's a good addition you might enjoy. For example in my game there was a very memorable set-piece at the end, just before my group left the Isle, where the port was invaded by pirates; the PC's had to help defend the place, which was pretty epic.
Lastly, there was also a stand-alone adventure published in Dragon Magazine, maybe a couple of years before Savage Tide? It started with the PC's being ship-wrecked on the Isle, and having to interact with some villagers and their Zombie Master, help them out to survive, etc. I ran it as a one-off, PC's around level 3 or 4, and the players really enjoyed it. I think it was in Dungeon 114, called Torrents of Dread.
