No lightning speed
No Adult Gon
Who wins?
Depends how the fight happens. I say if it is set for a specific date, lets say it is agreed that they will fight in a weak's time then Killua would win high diff coz of prep (Gon is also brilliant at prep, this was showcased at Heaven's Arena wen he created strategies for dealing with those nen using cripples after seeing them fight only once, but I believe this is more Killua's forte so he takes it.) If it's a spur of the moment fight then Gon stomps Killua, Gon is like Naruto in that he is good at spur of the moment strategies and is willing to take risks that normal people wouldn't to see them through, making him unpredictable. Plus there's janken which is a one hit KO and a one hit Kill if he happens to use scissors, also makes it unpredictable because Gon developed a way to turn the weaknesses of Janken into strengths thanks to his matches with Knuckle. Yoyo's aint gonna do shit to Gon and sharp claws aint doing shit either.
If Godspeed were allowed though than Gon gets stomped, he won't last even a second, literally, he won't last even a literal second. Adult Gon stomps any form of Killua.