Small Engines (Lawn Mowers, etc.): Briggs 18 HP Vanguard 422707, aluminum block, center punch

Could you tell me if oversize valve seats are available.

Sorry Ralph but the answer is no.
If the valve seat has came out you can sometimes make it stay in  by spiking the aluminum around the seat with a punch. Go around the seat in  starburst pattern. Put your center punch as close to the seat as you can and dent the aluminum block. The ":spike" will fold aluminum over the edge of the seat and usually hold it in.
We occasionally make this repair in my shop, but, we offer no warranty on the repair.