Don't know why, but all the parts aren't listed on the web page if you use the cross reference. ALL parts for current-production GMS engines are carried. Here's the stock number and suggested retaip price for the .61 piston ring:

GMEG6370 611031 PISTON RING 61 RING $4.99

Also, if you type in "GME piston ring", it comes up. Or even just "piston ring". Their search engine really is very good.

Also...don't know who would say the engine won't be supported soon. It's certainly not slated for discontinuation any time soon, as far as we know.