We need the complete model of your cub.
But I've got a Cub and it's common for the clutch spring to fall off when the drive belt gets too loose.
You can download a parts lists that shows the spring to see where it attaches or remove the deck, elevate the mower and you can see where it attaches...
Be aware if you try to look for a Walbro LMT using the Briggs part number for your engine's carb you will be directed to the Nikki replacement carbs, so when searching ebay you will probably have to search for a USED Walbro LMT carb. (and look for the name Walbro LMT on the side of the carb)...