How would I make this Camera-Script follow the Torso again after the player dies?

You will need to get Player_Torso again every time the player respawns. Along with that, you'll also need to re-insert your BodyPositioner. To do that, you will have to listen to the CharacterAdded event of the player. Here's the rewritten code -- I'll annotate my changes.

local Camera = game.Workspace.CurrentCamera

local Player = game.Players.LocalPlayer
-- Get the player's character, or wait for a new one to spawn.
local Character = Player.Character or Player.CharacterAdded:Wait()
-- Renamed Player_Torso to Torso. It's uncommon style to use underscores in Lua,
-- especially when naming variables.
local Torso = Character:WaitForChild("Torso")

-- This function will be called every time the player's character spawns.
function InitializeCharacter(character)
	-- Update the Torso and Character variable
	-- so that the camera locks on to the player's current Torso.
	Character = character
	Torso = character:WaitForChild("Torso")
	local BodyPositioner = Instance.new("BodyPosition", Torso)
	BodyPositioner.MaxForce = Vector3.new(0, 0, math.huge)
	BodyPositioner.Position = Vector3.new(0, 0, Torso.Position.Z)
end

-- Make a connection to the CharacterAdded event.
Player.CharacterAdded:Connect(InitializeCharacter)
-- Initialize the Character we have right now.
InitializeCharacter(Character)

Camera.CameraType = Enum.CameraType.Scriptable
-- Unless you plan on using CameraSubject in some other way,
-- Setting the CameraSubject does absolutely nothing in a Scriptable camera.
-- Just leaving it in here just in case.
Camera.CameraSubject = Torso
-- Just set the camera's field of view once.
Camera.FieldOfView = 80

game:GetService("RunService").RenderStepped:connect(function()
	-- Make sure that the Torso hasn't been destroyed.	
	if Torso and Torso:IsDescendantOf(workspace) then
		-- This is more clear than your current code.
		-- Moves the camera 30 studs away from the torso.
		Camera.CFrame = CFrame.new(Torso.Position + Vector3.new(0, 0, 30))
	end
end)