the easiest way is too look at the date code. im assuming its a tecumseh engine. here is my example
    hmsk80 554321p  [85]123    this last bunch of numbers indicate what year   this example would indicate the year being 1985
 hmsk 554321p   05123  this would indicate 2005---1995----1985---1975--   the number 5 being the keyÂ