Muzzle is not a valid member of Tool?

I have this gun script that works fine for the default gun it is in, but whenever I try to put a different gun model (with all parts included to make it work), it says 'Muzzle is not a valid member of Tool' in the output. Please help.

local userInputService = game:GetService("UserInputService")
local tool = script.Parent.Parent
local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local character = player.Character
local characterState = character:WaitForChild("State")
local state = tool:WaitForChild("State")
local config = require(tool:WaitForChild("Config"))
local model = character:WaitForChild(tool.Name .. "Model")
local modelParts = {}
for _, v in pairs(model:GetChildren()) do
	table.insert(modelParts, {v, v.Transparency})
end

local canShoot = true
local clicking = false
local equipped = false

state.Changed:connect(function()
	tool.Remotes.ChangeState:FireServer(state.Value)
	if state.Value == "Aim" then
		if characterState.Value == "Sprint" then
			characterState.Value = "None"
		end
	end
end)

characterState.Changed:connect(function()
	if characterState.Value == "Sprint" then
		if state.Value ~= "Reload" then
			state.Value = "Sprint"
		end
	elseif state.Value == "Sprint" then
		state.Value = "None"
	end
end)

local function Shoot()
	tool.Muzzle.FireSound:Play()
	tool.Muzzle.FlashEmitter:Emit(1)
	tool.ShellEject.ShellEmitter:Emit(1)
	tool.Ammo.Value = tool.Ammo.Value - 1
	local bullets = {}
	for i = 1, config.Stats.Bullets do
		local spread = config.Stats.Spread * 5
		local position = tool.Muzzle.Position
		local direction = (CFrame.new(position, mouse.Hit.p) * CFrame.Angles(math.rad(math.random(-spread, spread) / 10), math.rad(math.random(-spread, spread) / 10), 0)).lookVector
		table.insert(bullets, {position, direction, config.Stats.Velocity + math.random(-20, 20)})
	end
	game.ReplicatedStorage.Events.FireBullets:Fire(bullets)
	tool.Remotes.Fire:FireServer(bullets)
end

local function Reload()
	if state.Value ~= "Reload" then
		state.Value = "Reload"
		tool.Remotes.Reload:FireServer()
		if tool:FindFirstChild("Magazine") then
			tool.Magazine.ReloadSound:Play()
			local mag = tool.Magazine:Clone()
			mag.Parent = character
			mag.CanCollide = false
			game.Debris:AddItem(mag, 10)
			tool.Magazine.LocalTransparencyModifier = 1
		end
		tool.Ammo.Value = 0
		wait(config.Stats.ReloadTime)
		if tool:FindFirstChild("Magazine") then
			tool.Magazine.LocalTransparencyModifier = tool.Magazine.Transparency
		end
		
		if equipped then
			if tool:FindFirstChild("Magazine") then
				tool.Magazine.ReloadFinishedSound:Play()
			end
			if characterState.Value == "Sprint" then
				state.Value = "Sprint"
			else
				state.Value = "None"
			end
		else
			state.Value = "None"
		end
	end
end

userInputService.InputBegan:connect(function(inputObject, processed)
	if equipped and not processed then
		if inputObject.UserInputType == Enum.UserInputType.MouseButton1 then
			clicking = true
			if config.Stats.Automatic then
				while canShoot and clicking and equipped and tool.Ammo.Value > 0 and state.Value ~= "Sprint" and state.Value ~= "Reload" do
					canShoot = false
					Shoot()
					wait(1 / config.Stats.RateOfFire)
					canShoot = true
				end
			else
				if canShoot and tool.Ammo.Value > 0 and state.Value ~= "Sprint" and state.Value ~= "Reload" then
					canShoot = false
					Shoot()
					wait(1 / config.Stats.RateOfFire)
					canShoot = true
				end
			end
		elseif inputObject.UserInputType == Enum.UserInputType.MouseButton2 then
			if state.Value ~= "Reload" then
				state.Value = "Aim"
			end
		elseif inputObject.KeyCode == Enum.KeyCode.Q then
			if state.Value == "Aim" then
				state.Value = "None"
			elseif state.Value ~= "Reload" then
				state.Value = "Aim"
			end
		elseif inputObject.KeyCode == Enum.KeyCode.R then
			Reload()
		end
	end
end)

userInputService.InputEnded:connect(function(inputObject, processed)
	if equipped and not processed then
		if inputObject.UserInputType == Enum.UserInputType.MouseButton1 then
			clicking = false
		elseif inputObject.UserInputType == Enum.UserInputType.MouseButton2 then
			if state.Value == "Aim" then
				state.Value = "None"
			end
		end
	end
end)

tool.Equipped:connect(function()
	equipped = true
	mouse.Icon = config.Cursor
	local motor = Instance.new("Motor6D", character.Torso)
	motor.Name = "TempToolGrip"
	motor.Part0 = character.Torso
	motor.Part1 = tool.MainGrip
	motor.C0 = config.Attachment
	
	if characterState.Value == "Sprint" and state.Value ~= "Reload" then
		state.Value = "Sprint"
	end
	
	for _, v in pairs(modelParts) do
		v[1].Transparency = 1
	end
end)

tool.Unequipped:connect(function()
	equipped = false
	mouse.Icon = ""
	if state.Value ~= "Reload" then
		state.Value = "None"
	end
	for _, v in pairs(modelParts) do
		v[1].Transparency = v[2]
	end
end)