Ken,
I am not sure exactly what you mean by end play. Do you mean that the rod is slipping back and forth on the crank pin and that is translating into piston up and down slop (especially at TDC)? If so, are you sure this slop isn't due to a loose socket fit on the piston ball? On my Sure Starts, the ball socket joint is very loose and needs tightening. My theory (I have a theory on everything!) is that Cox/Estes realized that the Sure Starts probably only ran a few times (if that!), so having the correct tightness in this joint really didn't matter to their typical customer base, yet would cost some effort (=$) to ensure that it was done just right!
Also I wasn't clear about the gasket you are talking about. When the crankcase is being screwed down to a metal tank, I think a gasket is required. However when you have the plastic reed retainer between the crankcase and the tank or backplate, I am not sure whether you need the gasket or not.
Finally, I thought the latest reed was some type of stainless steel. Larry Renger would know that best.