Precisely: he was physically merged with the Legacy, including the Weatherlight. You know, the airship whose powerstone contained an entire plane inside it so it would produce more or less infinite energy? Yeah, he's still got that. He lost Urza's spark (that was transeferred to him in Apocalypse) but the Weatherlight was almost a planeswalker itself in capabilities, just lacking the sentience to drive it. Karn has that sentience and all of the Legacy's capabilities.
Except the boys at MTGSalvation's Storyline forum have debunked the Legacy being an adequate substitute for a Planeswalker Spark. The Legacy itself is related to Karn the Released's abilities (Exiling things), but we've more or less confirmed that the Legacy is not the source of his spark.
This actually negated two of his possible ascension methods (the second being the Spark Transfer Device in the Mycosynth, which was destroyed when used). Currently, only three methods are left unaccounted for: Glacian's Spark, the Mirari, and Tezzeret's Clockworking abilities.
Glacian's Spark: Currently sealed inside the Might/Weakstones, it was never ignited when Glacian was alive. Because of it's location (sealed in a powerstone), the only problems with this theory are how it got ignited and why it didn't make Karn on par with Pre-Mending Walkers (as the spark wouldn't have been affected by the Mending). This is the most popular theory, and would take only a handwave from WotC to make it happen.
The Mirari: This thing is still capable of some very world-shaping powers, and it is possible that it could substitute for a spark if combined with the Legacy. The holes in this theory are simple, and almost immediately shoot it down: The Mirari was powered by Karn's Planeswalker abilities to begin with (while it was active on Dominaria, Karn was nearly powerless on Mirrodin). That's the major issue with the Mirari theory, and it isn't something WotC can retcon very easily because it would destabilize the OTG/OLS block storyline.
Tezzeret's Clockspinning: IMO, this is the most likely theory. It takes no more background, as it was "spelled out" during Test of Metal/Agents of Artifice. We know that this power is almost on-par with Pre-Mending Planeswalkers.
The holes in the third theory are very simple:
It's freaking Shenron-level Wishing power, and an asspull bigger than Bleach.