I have an old 2009 model 66cc engine that has 6mm studs 47mm bore and also has a smaller bolt pattern for jug/head, it also has a flat top piston and a straight plug head that has a much smaller combustion chamber, it was a Boygofast motor that was labeled as a Red Bat Engine, probably not to many of those still out there but it is an odd ball compared to the 66cc engine we have now, it has been an excellent engine as far as longevity, but it will vibrate your socks off at higher rpms.....lol
Measure the bore and you'll know 100% what you have but most likely if it has 6mm studs it is likely to be a 49cc and 8mm being most likelya 66c, qgain measuring the bore is the best way to be certain.
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Peace