How can I make that function only works when the player has a Shirt named "shirt"?

So, basically the script is to morph the player into Sasuke (anime character) and it already works. But I want to make it only works when the player has a Shirt named "shirt". That's the script

local Tool = script.Parent;
local SasukeClassicShirt = game.ReplicatedStorage.Clothes.SasukeClassicShirt:Clone()    
local SasukeClassicPants = game.ReplicatedStorage.Clothes.SasukeClassicPants:Clone()  
local SasukeClassicFace = game.ReplicatedStorage.SasukeClassicFace:Clone()

Tool.Equipped:Connect(function()
    player = script.Parent.Parent
    player.Humanoid.MaxHealth = player.Humanoid.MaxHealth + 9999999
    player.Humanoid.Health = player.Humanoid.MaxHealth
    player.Shirt:remove()
    SasukeClassicShirt.Parent = player
    player.Pants:remove()
    SasukeClassicPants.Parent = player
    player.Head.face:remove()
    SasukeClassicFace.Parent = player.Head
    local SSJHair = game.ReplicatedStorage.SSJHair:Clone()
    SSJHair.Parent = player
    SSJHair.CFrame = player.Torso.CFrame
    local Weld = Instance.new("Weld")
    Weld.Name = "SSJ"
    Weld.Parent = player.Head
    Weld.Part0 = player.Head
    Weld.Part1 = SSJHair
    Weld.C0 = CFrame.new(0.05, 0.5, 0.4) * CFrame.Angles(0,math.pi,0)
    Weld.C1 = CFrame.fromOrientation(0, -179.2, 0)
    Tool:remove()
end)

All you need to do is add a simple conditional statement after your Equipped event. Also, note that :remove() is deprecated. Use :Destroy() instead.

local Tool = script.Parent;
local SasukeClassicShirt = game.ReplicatedStorage.Clothes.SasukeClassicShirt:Clone()    
local SasukeClassicPants = game.ReplicatedStorage.Clothes.SasukeClassicPants:Clone()  
local SasukeClassicFace = game.ReplicatedStorage.SasukeClassicFace:Clone()

Tool.Equipped:Connect(function()
	player = script.Parent.Parent
	if player:FindFirstChild("shirt") then
		player.Humanoid.MaxHealth = player.Humanoid.MaxHealth + 9999999
		player.Humanoid.Health = player.Humanoid.MaxHealth
		player.Shirt:Destroy()
		SasukeClassicShirt.Parent = player
		player.Pants:remove()
		SasukeClassicPants.Parent = player
		player.Head.face:remove()
		SasukeClassicFace.Parent = player.Head
		local SSJHair = game.ReplicatedStorage.SSJHair:Clone()
		SSJHair.Parent = player
		SSJHair.CFrame = player.Torso.CFrame
		local Weld = Instance.new("Weld")
		Weld.Name = "SSJ"
		Weld.Parent = player.Head
		Weld.Part0 = player.Head
		Weld.Part1 = SSJHair
		Weld.C0 = CFrame.new(0.05, 0.5, 0.4) * CFrame.Angles(0,math.pi,0)
		Weld.C1 = CFrame.fromOrientation(0, -179.2, 0)
		Tool:Destroy()
	end
end)