As mentioned, I went through a similar exercise sizing carburetor for my 50cc 5-cylinder (methanol fueled) radial this summer. The plans specified a 2-stroke glow (Magnum XL-15A). I think Magnum was a clone of an OS but thatโs ancient history now, itโs no longer produced. To complicate matters I discovered I managed to buy the wrong equivalent to this carb, years go now. Glow engine selection is shrinking dramatically.
I noticed that sport engine manufacturers sometimes use the same carb model for displacement ranges like .25 & .35 or .25 & .15 CI. Caution, orifice sizes can vary significantly on engines of similar displacement depending on many variables. A 0.20 can be anything from 4-7mm depending on race/sport/boat engine, how much (if any) needle valve assembly occurs through the throat, engine timing, tuned pipe, pressurized fuel.... etc.
Anyways, I went to the Sceptre website & did a tabulation of OS 4-stroke engines orifice sizes. My simple logic was if it worked for those engines it should work for mine. Recognize that these are all single cylinder methanol glow engines. Carbs are feeding multi-cylinders are likely flowing at slightly higher net rate but I have no calculation method to support how much to factor that. I have a feeling the multi-cylinder engines actually may have had slightly smaller carbs & if so, may be a function of idling/transition characteristics over maximum rpm. Running gasoline may well be different sizing criteria. Anyways, here are my plots for your viewing pleasure.
example Sceptre link