The danger of taking your crankshaft dimensions off of a glow engine is that the stresses on a typical glow engine crank are so very different from those on an inline engine. Basically, the crank pin carries no torsional load at all, and the only torsional load carried by the crank main bearing is from a single cylinder.
A multi-cylinder crank carries torsional loads from the back of the engine to the front, and it needs to be stiff in bending, and it's long so it'll "ring" in torsion (yes, all discussed already -- I'm going to say it anyway!).
If you're going to copy something, copy a multi-cylinder engine. Even the model A engine -- which produced only about 1/5 horsepower per cubic inch, and needed an overhaul every 10000 miles -- had mains and big ends that were between 1/3 and 1/2 the cylinder bore.