9-B Saitama has Wall level durability and Wall level attack potency, and he has superhuman speed, meaning that he can traverse all 10 meters between Senku and himself within less than a second.
Senku normally has Human level durability, but with 2 hours of preparation, he has Wall durability via carbon fiber, which he has readily available, and most likely even higher durability. He is not going to get One-Punched by 9-B Saitama. Senku also has base Wall level attack potency, but because he has 2 hours of preparation, he can prepare himself and bring weapons that very likely exceed Wall level attack potency. He can bring different types of bombs, he can create primitive tranquilizers, since we know he is skilled in chemistry and he has a wide range of already made chemicals in his village, and chances are he also will bring some other weapon, possibly melee, since it would be useless to bring a ranged weapon in a battle where Saitama would blitz him.
So, assuming both take the optimal strategy and nothing else being taken for granted, Senku can OHKO Saitama, whereas Saitama will not OHKO him, but Saitama can blitz Senku, which Senku cannot do to Saitama. It ultimately comes down to whether Senku's tranquilizer and weapon can come into effect before Saitama can deliver enough punches to wear out his stamina, but based on the fact that Senku has extraordinary genius level intelligence and has 2 HOURS of preparation, in the most likely of scenarios, Senku will optimize for a plan that will ensure he can out-endure Saitama. And once Saitama has been tranquilized, Senku can tie him up or simply find some way to render Saitama unconscious fro longer, if not simply kill him.
CONCLUSION: Senku wins with High difficulty. (He wins, but he will definitely sustain AT LEAST some moderate injuries)