Small Engines (Lawn Mowers, etc.): B & S engine, exhaust valve seat, twin cylinder engine

Question
I recently acquired a John Deere 116 lawn tractor that is not running.  Was told by the previous owner that he was using it when it backfired and quit running.  The motor is a B&S 16 hp opposed twin cylinder engine model 402707 Type 0148-01 code 85051612.  Started disassembling the motor by first removing all the sheet metal and heads.  Found the exhaust valve seat on the left side to be loose (still in tact, but comes completely out of its machined groove in the block when the exhaust valve is open).  Was wondering it this would be the cause of the engine to stop running and if there is a way to repair this valve seat, at home, by a fairly competent homeower.  Please provide some direction as to whether or not this is worth trying to fix or should I send this motor to the scrap pile.  I have already checked and found out that a replacement shortblock is not available and the rest of the mower is in above average condition.   Thanks

Answer
Hello Keith:

Scrap the Engine. I have Several 402700 Series Engines in the Scrap Pile Now for the Same Reason as you Described. I have Talked to Briggs About the Valve Seat Problem and was Informed the Problem was Corrected. I have Tried Using Oversized Valve Seats to Correct this Problem and within a Year of Running the Valve Seat Still Loosened and Started Flopping Around, Causing a Miss at First and then Finally the Engine Stops Running. I Usually Strip the Engine of Parts that are Still Good (not the Internal Parts) and then Scrap the Block. Cylinder Heads, Coils, Carburetors, Stators and ect are Usually Useable and are Just as Reliable as New Parts, but the Used Parts are Usually Sold for Half the Cost of a New Part. If you Hit a Snag or this Does Not Correct the Problem,  I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Comments and Ratings are Greatly Appreciated. Thanks.

Respectfully

John