The way I remember it for myself is fairly easy.

10cc = .61 ci.

so... a .91 would be 15cc,

And....a 1.20 would be 20cc,

............a 1.80 (like in Moki, Saito, etc) would be whoppin 30cc.

And......that means that a 50cc engine comes out to 3.00 ci. Give/Take on these, as always, actual displacement is usually on the side of the cylinder of bigger engines in cc's, the formula given in above post would probably get you alot closer to what it is, but this is simple way here.