ORIGINAL: Don B.H.
...After the Shoe String build I ordered some .15 size birds and now I hear the .15 may be to large and I have about 16 of these engines.. Oh well........
Don, which engines do you have?
Most of the old profile planes from the fifties that called for a .19 - .35 can be flown with a modern .15. Bear in mind that the old engines were mostly plain bearings without mufflers so they were also lighter than the same sizes today. This has no meaning if you just want to bore holes in the sky or go round-n-round...but if you want to stunt, weight becomes important.
An older airplane calling for .09 - .19 on the other hand would work well with a modern .10 or maximum .15.
George