McCulloch Mc 91  ... 1966 Bore 2.165 ... Stroke 1.635 ... Displacement 6.06

Mc 91 Model 400165  10- IPL No 84456 / 87697 Owners Manual 85205 Production Period 12/66 to 4/68

This block is a little rough but may help in identifying by the casting what your block may be.