While searching the web I came across FP Smith tractor parts.They seem to be a valuable resource for old machinery. 
www.fpsmitheq.com/vintage.html

CrawlerAddict 

I will be posting a link for eBay for d2 and d4 pony motor replacement piston rings. The are not cat parts but work. The rings are a chrome composition for compression . They fit standard pistons and there is a plus . 010 set for slightly worn standard bores.

The are compression and oil for both standard and plus . 010. The seller is in the process of setting up a link for the cat part numbers. He worked with me when nobody else would.

The parts are not for the intended application, but work and fit. I will keep you posted. 

And no I will not make a dime, just sharing because I hate when i was told to junk it, pony motor.