Actually I don't think you need script for this because you could switch to a sprite whose max speed is higher and use special functions and inventory to control how long that lasts before another function is activated that switches you back to the normal sprite. But to implement it in script, you just need to have another counter to check in OnAfterMoveSprites; when it times out, you set the MoveSpeed back to 10.
It should be very easy to represent the countdown as a bar -- you already have it. Just make a global function that subtracts 1 from it every frame. You really don't need script for that.
Also, I see that you are using an Alter Inventory type function with your nitro boost -- why not just set the function to use 1 "Speed Boost" item. Then it will automatically not activate if you don't have any, and the action of the function can be "Raise Event" or whatever you want.