There is a section in an old Briggs manual about replacing the valve seats. I've done this a couple times on old engines and it does work. Basically pull the valve, put the seat back in as it should be, get the valve back in with spring tension to hold the seat in place. I take a pointed punch and go around the top of the seat area and displace metal to lock that seat in place. 8 or 10 punches will do it. The engines I did this to died of other things before the seat let go again.