12-15 years ago, I got piston rings for a HH70 Tecumseh from Hastings Piston rings. NLA from Tecumseh.
They have (or had) a website. In my case I called them with my bore diameter, my ring width dimensions, and the I.D. of the rings. they also helped me out with a set of rings for a Minneapolis Moline that had aftermarket .030 over size pistons. (MM never listed a .030 undersize bore for their 206.)
You can get the I.D. by measuring the thickness of the ring and subtract two times that number from the Bore dia. Or measure the depth of the groove in the pistons.