Crankshaft is 3/16 so normally it would be a 3/16 square key (3/16" x 3/16" x 3/4" long). Confused on how you could lose the key since they are built-in to the 4 spline and into the fixed sheave pulley next to the engine? If it is the driven unit on the jackshaft then you want a 3/16" key that is 2" long.