How can i change sprinting script to make it so that it only works when i hold down left control?

If you made this script, then just use UserInputService's InputEnded event. This event fires whenever the input service is not active anymore, in your case, when the left control key is not held down anymore. I also the gameProcessed parameter so your player's walk speed doesn't change when the player is typing.

local player = game.Players.LocalPlayer
local humanoid = player.Character.Humanoid

local UIS = game:GetService("UserInputService")

UIS.InputBegan:Connect(function(input,gameProcessed)
    if gameProcessed then return end
    if input.KeyCode == Enum.KeyCode.LeftControl then
        humanoid.WalkSpeed = 35
    end
end)

UIS.InputBegan:Connect(function(input,gameProcessed)
    if gameProcessed then return end
    if input.KeyCode == Enum.KeyCode.LeftControl then
        humanoid.WalkSpeed = 16
    end
end)

We don't really help with free models, but you can change LeftControl to LeftShift on line 7.