Sears used several engines on their tractors over the years. My best guess would be one of three: 1) Tecumseh, 2) Briggs and Stratton, or 3) Onan. About the only way to find out for sure is to go to a Sears Service Center, and see if they can look it up there. Once you find that out, any equipment dealer that is a dealer for that specific engine can get you a repair manual for the engine.
Wish I had a better answer for you, but hope this helps out.
Stark Street Lawn and Garden
>I have an old ST/16 garden tractor that is starting
>to act up. I'm looking for an engine repair manual.
>The engine is labeled as a Craftsman, but I can't
>believe that they actually built it. Any idea what
>manufacturer actually made it?