cigron, there's nothing to gain by only using one ring, but plenty to lose. (Power.)
Rings aren't expensive. (Did the ring break 'in service' or while disassembling/assembling?)
motorpsycho, what you said isn't quite true. The rings are only at the top of the piston. The bottom, (skirt), contacts the cylinder wall. (Check out the scuff marks on an old piston.)
Rings are available in the US from GasBike.net,
here, (Piston + Rings $11, 48cc or 66cc),
or in Oz from ZBox,
here, ($7 for 48cc and $9 for 66cc rings).
To fit them, Vistaman73, use your thumbnails to carefully expand the ring until it
just slides over the piston. Ensure that the locator pin in the piston groove is aligned with the ring's end-gap.
When sliding the barrel on, ensure that the gap stays aligned with the pin and compress the ring onto the piston with your fingers while lowering the barrel.