RE: How much motor to fly this?
If the wingspan is 64.5 inchs or close then the airplane pictured in your other thread is most likely the Sterling. What is pictured is definitely a Stearman and of that there is no doubt.
Its likely if the gentleman selling runs glow powered cars then it probably is a four stroke as that would be to hard to miss and because the engine is mounted its just to much trouble for him to remove it to see exactly what engine it is.
Ok I do have a hard working Stearman that earns its living pulling gliders aloft on a tow line (heaviest tow to date was a glider just over 7.5 pounds. This is the Great Planes Stearman PT-17 that is a larger ship with almost ten inchs greater wingspan on the top. For that ship I use an OS .120SE (A current model).
For that Airplane I would use no more than a 91 in a four stroke (even that may be a bit much) and to be honest my preference for that ship would not be a fourstroke at all but the current OS .75AX two stroke.
If it is a Sterling then I would not trouble yourself looking for original Sterling CG suggestions and such as this ship is from a time that modelers were expected to make their own decisions and I doubt there is much info there and even may be in doubt.
Instead use one of the online biplane programs for finding a reasonable CG or the collective averaging technique that has served well before computers. Definitely would not use a CG any further back than 25% MAC and I would take a conservative approach to throws.
John