Standard Piston?

look here for rings
www.totalseal.com
-Duck

If were unable to get them, I do believe the 0.5mm oversize rings would survive, set to the correct gap. However it looks like you are on the right track with the standard rings.
RegardZ.

tellietubbie... Sometimes folks ask questions that are sort of outside the box. If other folks have used the rings you refer to, they must be able to work. 50cc may actually be a tad less and the rings may not distort sufficiently to hurt sealing so if you go with the 550 rings, I can't say how you would trim 3 piece oil scraper rings (the bottom ring). they usually don't get trimmed so just install as you normall would. As to the upper and center rings, get your 500 cylinder and place the rings into the bottom of the bore; about 1/2 from the bottom. There is the least wear here and original bore dimensions closest to what they originally were. Measure the gap, if possible and adjust the gap by using a file to whatever the recommended spec is for your model. Use a small ignition type flat file and take only a little material and remeasure frequently. If you don't have sufficient gap when initially attempting to measure, you will have to remove some material from both ends of the ring and check to see if you have sufficient end gap to actually make a measurement.