The book I have says 57-64 cc. I have a #178 head and a #234 head if I remember correctly. When I took the engine apart all the bearings were original but on one cylinder. My guess is that something let loose and smacked a head. One head was replaced.
Scratching my head. At 64cc and flat-top I came up with only an 8.3:1 compression ratio. Does that sound right to you ? Sure. It makes for a street engine that can run well on regular gas but it certainly limits power output.