Determining who wins between these two here is honestly a lot harder than determining Oga and Takamiya back in the day. In fact, such a decision is even harder to determine than Oga against Fuji.
Why?
Because we've seen Johan's capabilities. Takamiya was a wild guess if he'd beat Oga or not, and Fuji will too.
But this guy, I mean damn. He's had the better end of Oga since his proper debut in battle.
His quickness surpasses Hilda, as well as Takamiya, and even Oga.
Some people speculate that all of these feats are only because he has that little marble in his mouth. That his sheer quickness (teleportation) made him LOOK tough, but in reality, without it, he ain't gonna be better than Oga.
But remember, we've seen him BLOCK OGA'S ATTACK WITH HIS BARE HANDS before. So that is proof enough that his teleportation is not what his limits reach.
As for Oga, we haven't necessarily seen him fight Johan in his brand new CDS (Complete Daddy Switch) Mode yet. And I'm sure he's gonna have to use that to even properly fight Johan.
In terms of using pure plot as well, does it not favor Oga. Yeah, usually plot terms Oga will get the win, but because he was supposed to fight Fuji, not Johan, makes this less likely he'll win. Because while Fuji is put in high regard Johan is still the one pulling the strings. Why would the plot go with a higher-up losing before someone lower? If Oga can beat Johan, it's 99% certain he'll beat Fuji, and the author wouldn't want that predictability.
What I believe honestly is probably an interruption, in order to save faces for both of them: they both remain undefeated. Johan will probably leave to do other work, but this fight may last 3 chapters max, for the author to show how far the limits of Oga and Johan's powers are.
So, who do you think will win? Or does the smell of interruption bode well?