Team Kha'Zix ftw.
In-lore though, I do think Kha'Zix would win.
First, you say the battle ended in a draw, and though that's correct, let's look at the aftereffects. Both were nearly beaten to death, but Kha'Zix fully recovered, now showing no damage at all. This suggests that after the battle, Kha'Zix continued to hunt and feed, rejuvenating his form to full might and perhaps even gaining some new traits along the way. Rengar on the other hand was left on the verge of death, out of commission in his den. Plus, where Kha'Zix wears not a single scratch from the fight, Rengar completely lost an eye.
Second, Kha'Zix continually evolves and changes. While Rengar is probably only sharpening the tools and honing the skills he already has, Kha'Zix can adapt entirely new abilities, not only growing physically stronger but perhaps learning as well. And since Kha'Zix is all about adaptation, since he's already seen what Rengar is capable of, Kha'Zix can come fully prepared with a slew of skills and traits. Basically, the Voidreaver is much more adaptable and willing to change should he be met with failure, while the Pridestalker is limited in his arsenal and capabilities. Hell, he's called the Pridestalker; his arrogance will probably make him too stubborn to change even if it's costing him his life.
Third, if it's a match of pure strength and will, let's just look at their origins.
Rengar was born and raised on Runeterra and hunts the strongest creatures this world has to offer. Kha'Zix crossed through dimensions from one much more horrific than this one and started frail in an alien world yet still managed to become an alpha predator here.
Rengar is limited by the laws of one world; Kha'Zix has the strength of two. Rengar was raised by humans to be a hunter; Kha'Zix was born to be the apex predator. Rengar most likely hunted simple animals; Kha'Zix, pre-evolved, probably had to deal with the likes of beings like Cho'Gath and Rek'Sai. Rengar had the care and assistance of his foster parents in his early years to become a hunter; Kha'Zix came through dimensions alone and frail, yet still managed to become a fearsome beast without anything more than his bare claws and teeth.
Not only is Kha'Zix's will stronger and he's better suited to dealing with threats based on experience alone, the Voidreaver is even more in inspirational. He's had to deal with much more, yet still becomes the top of his game.
Is Rengar a worthy opponent for Kha'Zix? Absolutely, I don't doubt he'd be able to inflict some pretty gruesome wounds on the Voidreaver. But I think where Rengar can hurt Kha'Zix, Kha'Zix can ultimately kill Rengar.
It's a battle between one who's incredibly strong against another who's strength can only grow. And if they were already matched back then but Rengar couldn't kill his rival, then the balance of power between the two can only shift more and more towards Kha'Zix.
ganks
GET TO DA CHOPPA! DO IT NAUW!
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. He's only agile when he's in brush (what is he doing? Smoking the stuff?), while Kha'Zix can leap great distances. 
