Nice looking Senior. The frame number is stamped on l;eft side of the plate connecting the fork and frame, near the top. Original engines in the 6 HP Senior C were Continental Model 1120, Spec No B400717. Wayne Mahaffey sells a repro kick starter for this engine. Original engines in 4HP Compacts were Continental Model AU10, Spec No B400554. They are basically the same engine. 1966 and later Seniors used Tecumseh Model H60 engines. Good luck.