How to make parts turn visible/invisible when the player is holding a tool and hovering over it?

Hello, Adv3rtizement!

Use mouse.target to get the part mouse is pointing to =D, so don't need to use Rays(they're not really precise)

Using Ray(not precise)

player = game.Players.LocalPlayer
handle = script.Parent
tool = script.Parent.Parent

local ChangedPart = nil

tool.Equipped:connect(function(mouse)
    while tool:IsDescendantOf(player.Character) do -- Loops while the tool is in the player's character (equipped)
        wait(0.1)
        ray = Ray.new(tool.Handle.CFrame.p, (mouse.Hit.p - handle.CFrame.p).unit * 30)
        -- Makes the rays
        part = workspace:FindPartOnRay(ray, player.Character, false, true) 
        -- finds the part
        if part then -- checks if it found a part.
			if ChangedPart then
				ChangedPart.Transparency = 1 --Backs the transparency to 1
				ChangedPart = nil
			end
            if part.Name == "Hitbox" and part:IsDescendantOf(workspace.NPCs) then 
            -- checks if the part is a hitbox and it is a descendant of the NPC folder.

		ChangedPart = part --Stores the part it's changing....

                part.Transparency = 0 -- Changes part transparency
            else
                print("Part Not Hitbox") -- If the part isn't a npc's hitbox
            end
        else
	    if ChangedPart then --Tests if had a part
	   	ChangedPart.Transparency = 1 --Backs the transparency to 1
		ChangedPart = nil
	   end
            print("Not part") -- If the ray didn't get a part.
        end
    end

end)

tool.Unequipped:connect(function(mouse)
	if ChangedPart then
		wait(.1)
		ChangedPart.Transparency = 1
		ChangedPart = nil
	end
end)

Using mouse.target

player = game.Players.LocalPlayer
handle = script.Parent
tool = script.Parent.Parent

local ChangedPart = nil

tool.Equipped:connect(function(mouse)
    while tool:IsDescendantOf(player.Character) do -- Loops while the tool is in the player's character (equipped)
        wait(0.1)
        part = mouse.target -- mouse.target is the part the mouse is pointing to....
        -- finds the part
        if part then -- checks if it found a part.
			if ChangedPart then
				ChangedPart.Transparency = 1 --Backs the transparency to 1
				ChangedPart = nil
			end
            if part.Name == "Hitbox" and part:IsDescendantOf(workspace.NPCs) then 
            -- checks if the part is a hitbox and it is a descendant of the NPC folder.

		ChangedPart = part --Stores the part it's changing....

                part.Transparency = 0 -- Changes part transparency
            else
                print("Part Not Hitbox") -- If the part isn't a npc's hitbox
            end
        else
	    if ChangedPart then --Tests if had a part
	   	ChangedPart.Transparency = 1 --Backs the transparency to 1
		ChangedPart = nil
	   end
            print("Not part") -- If the ray didn't get a part.
        end
    end

end)

tool.Unequipped:connect(function(mouse)
	wait(.1)
	if ChangedPart then
		ChangedPart.Transparency = 1
		ChangedPart = nil
	end
end)

Hope this helps

Good Luck with your games