I have a Yard King Performance model 229610x89a with a Briggs and Stratton 6 hp quantum engine lawn mover. It is a powered propelled walk behind mower.
When I fill the tank over 3/4 full, it leaks. I'm thinking the gasket is bad, however, where do I get a new one and will that fix the problem?
It all started when I fell on the mover. I must have broken the gasket seal?

It is more likely that you cracked the tank and patching isn't really an option. You can replace the tank but try to get it from the manufacturer, local shops won't be much help.

Your tank is probably cracked. These tanks get cracked easily.