Point 1: Shinigami (Death Gods, Soul Reapers, whatever) cannot be hurt by non-zanpakutou.

Point 2: Shinigami cannot be perceived by humans without powerful spirit senses. As such abilities are nonexistent in Himura Kenshin's world, it can be thus postulated that Kenshin cannot see Ichigo.

Point 3: The Shinigami Shunpo (flash step) can exceed anything a standard human can use in terms of movement speed.

Point 4: Ichigo's bankai form is excessively fast, and while Kenshin is extraordinarily quick for a human, he is still a human.

Point 5: Ichigo's hollow form is yet stronger and faster still.

Kenshin loses. Badly.