Heres a pic of my engine[although its not an OS],but this post does deal with the adaptor.I made the adaptors on my lathe.The carb is a 8-9 MM i blieve.Its the same size as the throat on the aaptor,maybe a little larger.
Anyway,its a WA59 and believe its off a 31cc weedie.The reason i like it is it has the stubby needles with the large screwdriver slot,and it has the large triangular shaped throttle plate for an easy hookup with at least to different styles.You wont find the number but it has a 20 stamped in the throat.All small walbros use the same distance on the bolts.You can use any carb you want.I looked at a couple i have here.The one that i would suggest first would be a WT160-B.It comes from small stihl power equipment,weedies.
The one adaptor on the right of the pic is one i made for my ASP .91 but i learned that ABC style engines wont work right and that i needed a ringed engine.So in short conversation,thats an extra that im not going to use.Im going to send you a PM.The bottom dimension of the adaptor is 14-15MM.Its already drilled and tapped for a walbro using 8-or 10x32 bolts.You cant really see it but the back side of the adaptor is flat so it will sit close to the cylinder with out hitting it.Im just posting this info so it might help.
OOOOOPPPSS!Here they are.