Your engine is 25 cubic inches (410 cc), fourth design generation, with vertical crankshaft and flo-jet carburetor. It has plain main bearings and a 12 Volt geared-type electric starter with alternator. Here is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6nlyHVJ1DajI.pdfHere is the operator's manual:
http://bsintek.basco.com/BriggsDocumentDisplay/msoBGL-K_ny7tH217FzoPu.pdfBecause your engine is fourth design generation, the usual overhaul manual does not apply. You will need the late side-valve single cylinder manual, which is known by its Briggs part number of 270962.
I do not know of any reason why there would not be a market for the engine, so long as it was in good operating condition. As Bob said, if it has sat in that creek with water in the cylinder, it is only usable for spare parts. The same applies if it is in poor mechanical condition.
