Cylinder question

1
All cox cylinders will fit all cox engines.  Ok, maybe not exactly with the major size differences, but the .049
.050 and .051 are interchangeabe between TD and reed engines. The piston must match the cylinder .049, .050, .051 pistons are different diameters.

2
Not all pistons are tapered.  The newer pistons are not tapered.  If you have a piston and cylinder loose, very gently push the piston up past the normal top dead center position.   If the piston becomes tight in the cylinder there is "pinch" because the cylinder is tapered and the skirt of the piston flares.  If the piston passes either the piston or cylinder is straight walled.  Since tapered cylinders usually only allow the pistion slightly past the head seat, and in my experience practically all pistons have a larger skirt, it should be obvious.  Be gentle as you can jam a piston in the cylinder with little force.

3
https://en.m.wikipedia.org/wiki/Cox_model_engine
There is a chart halfway down that offers some guidance for number stamped cylinders.

Phil