Clean the pocket where the valve seat rests. Gently hammer valve seat back into the head with a proper fitting socket. Then you have to use a punch to push metal towards the valve seat ,all the way around the seat. Called restaking the seat I think. Should be several videos on youtube about it.
Likely cause is the cylinder was overheated. Most people do not take the starter/blower/flywheel cover off and clean the cylinder block cooling fins. Engine could have a lot of oil, dirt, grass and bugs that clog the cooling fins. It's in the engine manual to check like yearly. Normally happens on the exhaust side as it runs hotter than the intake valve.
slomo