Considering all the Chinee did was to bore the 50's out to 66cc's you would have a very hard time determining the size of these motors. I've had both sizes and there's no difference externally I could determine. The majority of the parts are the same except for pistons, rings, and jugs. There are some 66's with the 6mm studs, too, further confusing the situation.
Oh, yeah, The "Oregon" plug means that it's from the Oregon chainsaw company. Their chainsaw bars and chains are world famous.