Is there a way to make this script turn into an OnTouch?

You could use a table(local Table = {}) and insert the Player into it(table.insert(Table, Player)).

Example:

script.Parent.Touched:connect(function(Touched) -- Once the part is touched
	local Table = {} -- The table
	if Touched.Parent:FindFirstChild("Humanoid") then -- Make sure it's a player
		table.insert(Table, game.Players:FindFirstChild(Touched.Parent.Name)) -- Insert the player into the table
	end -- End the if statement
end) -- End the Touched function

And then replace SetWalkSpeedForPlayers(game.Players:GetPlayers(), 0) and SetWalkSpeedForPlayers(game.Players:GetPlayers(), 16) in your code to SetWalkSpeedForPlayers(Table, 0) and SetWalkSpeedForPlayers(Table, 16).

Example, and full script:

local function SetWalkSpeedForPlayers(Players, WalkSpeed)
	for _, Player in pairs(Players) do
		Player.Character:WaitForChild("Humanoid")
		Player.Character.Humanoid.WalkSpeed = WalkSpeed
	end
end

script.Parent.Touched:connect(function(Touched) -- Once the part is touched
	local Table = {} -- The table
	if Touched.Parent:FindFirstChild("Humanoid") then -- Make sure it's a player
		table.insert(Table, game.Players:FindFirstChild(Touched.Parent.Name)) -- Insert the player into the table
		SetWalkSpeedForPlayers(Table, 0) -- Set the walkspeed for the players in the table
		wait(5)
		SetWalkSpeedForPlayers(Table, 16) -- Set the walkspeed for the players in the table
	end -- End the if statement
end) -- End the Touched function

There probably is a better way to script it, but this way works(or at least, it did for me..)

Sorry I can't explain it better.