Actually the long shaft versions work out better because the diameter of the shaft is greater and you generally have a single piece crankshaft with no extension studs like you have on a short shaft engine.

It doesn't make a whole lot of difference. Its six of one and a half dozen of the other.

On the Ryobi, the latest ones are two ring.

Either cylinder type will hold up fine for model use. The double ring type has a little more compression. Of the single rings, I think there are thick and thin ones? Whichever type you come up with will work fine.

Enjoy,

Jim