Since your engine has the 6mm studs on the cylinder the only may for sure to know what size engine you have is to measure the bore, the 66cc engines will have a 47mm bore, if this is an older engine the 49 and 66cc both had 6mm studs. You can easily tell if you saw a 66 and a 49 next to each other since the 49 has a considerably smaller cylinder and head than the 66cc, I got a 49 at the shop I can measure across the head and post that measurement, the 66cc head is noticeably wider than the 49, the distance between the head bolts is also different, with those measurements you will be able to tell without having to measure the bore.