I have a craftsman 5 horsepower tiller about 25 years old. I've been told it probably has a Tecuseh engine in it. Where would the serial number be on the engine?

Hi,
It is probably stamped into the top of the blower housing and possibly covered by othe parts. When you find a number beginning with 143 it is a Tecumseh and I can cross it for you.
Peace,
Clarke