I am not familiar with Standard Monarch. If yours is a twin cylinder 2 wheel tractor, with perhaps a seat/sulky frame behind, I would search for 'Standard Twin' and see what comes up. See if the pictures are like yours.
You can probably get rings from Hastings Piston Rings. I got some for a Tecumseh once that were NLA as Tecumseh parts. Not cheap, but at least they were available.
You do have to know the bore of the cylinder, the depth of the ring grooves or the id of the ring and the width of each ring if you go this route.