Small Engines (Lawn Mowers, etc.): valve seat fell out 17.5HP B&S 31C707 0230-E1, craftsman lawn tractor, briggs stratton engine

Question
I have a Craftsman lawn tractor with a 17.5 hp Briggs & Stratton engine (model # 31C707, type number 0230-E1). 021106ZA01120 is the other barcode. Here is my problem.
Engine obviously must have heated and expanded causing the valve seat to fall out. So I removed the retaining clips vlave unburned etc. and pressed the seat back in. Still backfires out valve. Should I purchase a new valve or #5 cyl. head assembly and if so how do I find the date code ? http://shop.briggsandstratton.com/BShopProductListingPage.asp?rsvp=0&PARENTID=0&... shows 792360 Head−Cylinder (Used After Code Date 06011900). & ----Note---- 698148 Head−Cylinder (Used Before Code Date 06011900). Same ---Note--- for the valve/seat combo. It shows this part is replaced by  Part Number: 793991 HEAD-CYLINDER ... do you have any words of wisdom ?

Answer
Mike
Sounds like the valve seat and the valve need to be ground to match eachother.Also check the valve spring as it may be defective, I would not recommend using the old parts because if it goes bad again then you will have to take engine apart again and then do it right.
Thanks