That 100 means Honda "GCC" family (SCR100...) 50.0*52.0 ie. 81cc in GY6 (Stroke: 41.4) . Better ring is difficult to get in China.
72cc Φ47 1.0*1.0*2.0 Tons horizontal (pit bike/ cub...) also this size...
Φ44
http://www.dongya.cn/prodlist.asp?page=1&tp=4&keywords=
Generic GY60 is 1.0*1.0*2.0 BUT that shop IS using CB125T/CM125 (ie. those 125 twins): 1.0*1.0*2.5 instead (it MAY work...)