I'd wait on doing anything just yet. I know you put ATF in. Try some Marvel Mystery oil. Have you drained the crankcase yet? Is there any water in it? You may need to remove the head and put some persuasion to the piston. These Briggs engines are built to take a lot of abuse due to the type of people that tend to buy them. I feel that once you free it up it will be fine for years to come. You may need to do some light sanding on the bore once you free the rings up.
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